To my knowledge hudson and all its plugins should be able to run on java 5.

However the crap4j-plugin fails on java 5:
[CRAP4J] Collecting Crap4J analysis files...
[CRAP4J] Searching for report files within report.xml
[CRAP4J] Using the new FileSetBuilder
FATAL:
java.text.DecimalFormatSymbols.getInstance(Ljava/util/Locale;)Ljava/text/DecimalFormatSymbols;
java.lang.NoSuchMethodError:
java.text.DecimalFormatSymbols.getInstance(Ljava/util/Locale;)Ljava/text/DecimalFormatSymbols;
at
com.schneide.crap4j.reader.util.NumericalParser.<init>(NumericalParser.java:18)
at
com.schneide.crap4j.reader.util.NumericalParser.<init>(NumericalParser.java:13)
at
com.schneide.crap4j.reader.ReportReader.parseStatistics(ReportReader.java:142)
at com.schneide.crap4j.reader.ReportReader.parseData(ReportReader.java:131)
at
hudson.plugins.crap4j.Crap4JPublisher.loadProjectCrapBeans(Crap4JPublisher.java:134)
at
hudson.plugins.crap4j.Crap4JPublisher.createCurrentProjectCrapBean(Crap4JPublisher.java:116)
at hudson.plugins.crap4j.Crap4JPublisher.perform(Crap4JPublisher.java:96)
at
hudson.model.AbstractBuild$AbstractRunner.performAllBuildStep(AbstractBuild.java:372)
at
hudson.model.AbstractBuild$AbstractRunner.performAllBuildStep(AbstractBuild.java:360)
at hudson.model.Build$RunnerImpl.post2(Build.java:183)
at hudson.model.AbstractBuild$AbstractRunner.post(AbstractBuild.java:345)
at hudson.model.Run.run(Run.java:917)
at hudson.model.Build.run(Build.java:112)
at hudson.model.ResourceController.execute(ResourceController.java:93)
at hudson.model.Executor.run(Executor.java:119)

In the java 1.6 doc the class java.text.DecimalFormatSymbols is described as
available since 1.6
http://java.sun.com/javase/6/docs/api/index.html?java/text/DecimalFormatSymbols.html