groovyc compiler barfs on processing of simple groovy scripts when invoked from maven
your bug is a result of a conflicting antlr version from maven. We know this problem and currently we fork to work around this problem. It will be solved completly in near future by a different class loader handling. But please note that this is no bug, "just" conflicting jars.
I got this message when executing groovy helloworld.groovy. After some experimentation, it appears that removing checkstyle-all-3.5.jar from the classpath circumvented the problem
it's possible that checkstyle uses antlr. Anyway, looking at http://groovy.codehaus.org/Ant+Task+Troubleshooting should help a little too
Just for those that are interested, the problem still exists in the 1.0-JSR-04 release. It is still necessary to use the -all jar to get Groovyc to work under Ant/Maven control. That or the other solution on the above confluence link (untested as yet by me)