Drools-compiler currently depends on:
- org.antlr:antlr-runtime:3.3
- org.antlr:antlr:3.3  (= the antlr-compiler)
- org.antlr:gunit:3.3
- org.antlr:stringtemplate:3.2.1
- antlr:antlr:2.7.7

antlr 3.3 is messed up, because it depends on antlr 2.7.7. antlr 3.4 fixes that problem. Meanwhile antlr 3.5 is available too.
antlr 2.x and antlr 3.x use different package namespaces, so even though EAP 6 has antlr 2.x, it doesn't prevent drools from using antlr 3.x

Requirements:
1) Upgrade antlr from 3.3 to 3.5 (or 3.4 if you prefer that). Consequently, remove the antlr 2.x dependency.
2) Drools-compiler's runtime classpath should only depend on antlr-runtime, it should not depend on antlr (= the antlr-compiler). The maven-antrun-plugin which compiles the *.g files should have an extra dependency to depend antlr (= the antlr-compiler).