Now, it's early days for both Groovy and Grails for sure, but it's also a new language, something which the others can't claim (believe me, I am a big fan of them - particularly python, but new they are not). The newcomer groovy has some tricks to teach the old guys, and integrates superbly with Java and the JVM already.
It's got a way to go - there is an early access version of the book but otherwise documentation is extremely scarce. Even so, the builders, streaming markup and other ideas are fantastic. What it needs, apart from maybe some funding and development time, is an environment that lets people take advantage of the new stuff.
And so my plea is simple - if adding support for the other languages in netbeans, please include Groovy and Grails in the considerations.