Global AST transforms defined in same workspace (but different project) aren't run

I cannot run Spock specifications when I'm working on the Spock codebase. The global AST transform defined in project spock-core isn't picked up when running (for example) specs in spock-example, even though spock-example has a project dependency on spock-core. When I replace the project dependency with a Jar dependency, everything works fine.

To reproduce:
check out http://spock.googlecode.com/svn/trunk/
run mvn eclipse:eclipse
import existing projects into workspace
turn spock-core and spock-example into Groovy projects, add source folders src/main/groovy and src/test/groovy
build both projects
run a spec in spock-example (output: "class xxx is not valid Spock specification")