readlink is not provided by default on Solaris.
Notice java 7 now provides an API to deal with symbolic links.

Here is the stack trace obtained on solaris:
java.io.IOException: Cannot run program "readlink" (in directory "/opt/instance/ SUNWappserver/domains/dom-hudson/config/."): error=2, No such file or directory
at java.lang.ProcessBuilder.start(ProcessBuilder.java:460)
at java.lang.Runtime.exec(Runtime.java:593)
at org.jruby.ext.posix.util.ExecIt.run(ExecIt.java:61)
at org.jruby.ext.posix.util.ExecIt.runAndWait(ExecIt.java:51)
at org.jruby.ext.posix.JavaLibCHelper.readlink(JavaLibCHelper.java:196)
at org.jruby.ext.posix.JavaPOSIX.readlink(JavaPOSIX.java:160)
at hudson.Util.resolveSymlink(Util.java:1236)
at hudson.Util.resolveSymlinkToFile(Util.java:1166)
at hudson.model.Run.parseTimestampFromBuildDir(Run.java:346)
at hudson.model.Run.<init>(Run.java:294)
at hudson.model.AbstractBuild.<init>(AbstractBuild.java:182)
at hudson.model.Build.<init>(Build.java:103)
at hudson.model.FreeStyleBuild.<init>(FreeStyleBuild.java:41)
at sun.reflect.GeneratedConstructorAccessor313.newInstance(Unknown Sourc e)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingC onstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
at hudson.model.AbstractProject.loadBuild(AbstractProject.java:1120)
at hudson.model.AbstractProject$1.create(AbstractProject.java:317)
at hudson.model.AbstractProject$1.create(AbstractProject.java:315)
at hudson.model.RunMap.retrieve(RunMap.java:225)
at hudson.model.RunMap.retrieve(RunMap.java:59)
at jenkins.model.lazy.AbstractLazyLoadRunMap.load(AbstractLazyLoadRunMap .java:667)
at jenkins.model.lazy.AbstractLazyLoadRunMap.load(AbstractLazyLoadRunMap .java:629)
at jenkins.model.lazy.AbstractLazyLoadRunMap.search(AbstractLazyLoadRunM ap.java:368)
at hudson.model.AbstractBuild.getPreviousBuild(AbstractBuild.java:220)
at hudson.model.AbstractBuild.getPreviousBuild(AbstractBuild.java:103)
at hudson.model.Job.getBuildStabilityHealthReport(Job.java:1020)
at hudson.model.Job.getBuildHealthReports(Job.java:980)
at hudson.model.Job.getBuildHealth(Job.java:945)