And then point your p2 installer to the downloaded archive. You will then be able to install Groovy-Eclipse. We have only tested on 3.6 M6 and we will continue to ensure that it works on later milestones as they come out.
As always, your feedback is appreciated!
EDIT (June 30, 2010): the zip above is outdated and so I removed it. Please use the update site below instead: