Here is the post build output of the plugin:

Publishing Cobertura coverage report...
Publishing Cobertura coverage results...
FATAL: Unable to parse /opt/hudson/.hudson/jobs/Bob Pipeline/builds/2010-12-10_19-10-01/coverage7.xml
hudson.util.IOException2: Cannot parse coverage results
at hudson.plugins.cobertura.CoberturaCoverageParser.parse(CoberturaCoverageParser.java:89)
at hudson.plugins.cobertura.CoberturaCoverageParser.parse(CoberturaCoverageParser.java:52)
at hudson.plugins.cobertura.CoberturaPublisher.perform(CoberturaPublisher.java:262)
at hudson.tasks.BuildStepMonitor$3.perform(BuildStepMonitor.java:36)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:603)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:582)
at hudson.maven.MavenModuleSetBuild$RunnerImpl.post2(MavenModuleSetBuild.java:651)
at hudson.model.AbstractBuild$AbstractRunner.post(AbstractBuild.java:529)
at hudson.model.Run.run(Run.java:1363)
at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:349)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:140)
Caused by: org.xml.sax.SAXParseException: XML document structures must start and end within the same entity.
at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.createSAXParseException(Unknown Source)
at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.fatalError(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLScanner.reportFatalError(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.endEntity(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.endEntity(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLEntityManager.endEntity(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLEntityScanner.load(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLEntityScanner.skipSpaces(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanAttribute(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanStartElement(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(Unknown Source)
at javax.xml.parsers.SAXParser.parse(Unknown Source)
at javax.xml.parsers.SAXParser.parse(Unknown Source)
at hudson.plugins.cobertura.CoberturaCoverageParser.parse(CoberturaCoverageParser.java:81)
... 11 more
FATAL: Unable to parse /opt/hudson/.hudson/jobs/Bob Pipeline/builds/2010-12-10_19-10-01/coverage8.xml
hudson.util.IOException2: Cannot parse coverage results
at hudson.plugins.cobertura.CoberturaCoverageParser.parse(CoberturaCoverageParser.java:89)
at hudson.plugins.cobertura.CoberturaCoverageParser.parse(CoberturaCoverageParser.java:52)
at hudson.plugins.cobertura.CoberturaPublisher.perform(CoberturaPublisher.java:262)
at hudson.tasks.BuildStepMonitor$3.perform(BuildStepMonitor.java:36)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:603)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:582)
at hudson.maven.MavenModuleSetBuild$RunnerImpl.post2(MavenModuleSetBuild.java:651)
at hudson.model.AbstractBuild$AbstractRunner.post(AbstractBuild.java:529)
at hudson.model.Run.run(Run.java:1363)
at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:349)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:140)
Caused by: org.xml.sax.SAXParseException: XML document structures must start and end within the same entity.
at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.createSAXParseException(Unknown Source)
at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.fatalError(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLScanner.reportFatalError(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.endEntity(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.endEntity(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLEntityManager.endEntity(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLEntityScanner.load(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLEntityScanner.skipSpaces(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanAttribute(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanStartElement(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(Unknown Source)
at javax.xml.parsers.SAXParser.parse(Unknown Source)
at javax.xml.parsers.SAXParser.parse(Unknown Source)
at hudson.plugins.cobertura.CoberturaCoverageParser.parse(CoberturaCoverageParser.java:81)
... 11 more
FATAL: Unable to parse /opt/hudson/.hudson/jobs/Bob Pipeline/builds/2010-12-10_19-10-01/coverage9.xml
hudson.util.IOException2: Cannot parse coverage results
at hudson.plugins.cobertura.CoberturaCoverageParser.parse(CoberturaCoverageParser.java:89)
at hudson.plugins.cobertura.CoberturaCoverageParser.parse(CoberturaCoverageParser.java:52)
at hudson.plugins.cobertura.CoberturaPublisher.perform(CoberturaPublisher.java:262)
at hudson.tasks.BuildStepMonitor$3.perform(BuildStepMonitor.java:36)
at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:603)
at hudson.model.AbstractBuild$AbstractRunner.performAllBuildSteps(AbstractBuild.java:582)
at hudson.maven.MavenModuleSetBuild$RunnerImpl.post2(MavenModuleSetBuild.java:651)
at hudson.model.AbstractBuild$AbstractRunner.post(AbstractBuild.java:529)
at hudson.model.Run.run(Run.java:1363)
at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:349)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:140)
Caused by: org.xml.sax.SAXParseException: XML document structures must start and end within the same entity.
at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.createSAXParseException(Unknown Source)
at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.fatalError(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLScanner.reportFatalError(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.endEntity(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.endEntity(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLEntityManager.endEntity(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLEntityScanner.load(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLEntityScanner.skipChar(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(Unknown Source)
at com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.XMLParser.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.parsers.AbstractSAXParser.parse(Unknown Source)
at com.sun.org.apache.xerces.internal.jaxp.SAXParserImpl$JAXPSAXParser.parse(Unknown Source)
at javax.xml.parsers.SAXParser.parse(Unknown Source)
at javax.xml.parsers.SAXParser.parse(Unknown Source)
at hudson.plugins.cobertura.CoberturaCoverageParser.parse(CoberturaCoverageParser.java:81)
... 11 more

It reports that it is looking in the home dir of the Master. The slave in tis situation is configured with a home dir of /home/hudson not /opt/hudson.