I’m not sure if this is a known problem, but I wasn’t able to find it. Anyway, the problem is that when you ‘include’ (apply) a script which uses an external plugin, then you can’t refer to it by its name.
An example can be seen at http://code.google.com/p/rivoli/source/browse/gradle/sub/coverage.gradle - in line 29, we’d like to refer to the Clover plugin by its ID instead of by its class name.