After investigation into an issue with another plugin, it appears the plugins are being compiled with JDK1.6.0_21. Even though the pom.xml specifies a different configuration.
I have checked the manifests for the plugins since moving to Hyperic 4.5, they are all compiled with 1.6.0_21. This is a problem, since we tell customers that they should be able to run with 1.5.
It appears the JDK1.5 maven configuration is not be used when building.