This functionality has been fixed in the 3.5 stream of the Groovy Eclipse plugin. Or at least it is fixed when running organize imports from the editor. So, running Ctrl-Shift-O inside the editor, or running format after save will work. However, running Organize imports from the package explorer or any other view will not.
The reason for this is that we need a way to hook in special groovy functionality. When running organize imports from inside the editor, this is possible to do. Hwoever, we have not figured out a general way to co-opt JDT's organize imports when running outside the editor. It is worth raising a bug for this. http://jira.codehaus.org/greclipse
Before the release of 2.0.0M1 (before the end of September), we will port this functionality back to 3.4, but we are finding that not many people are using 3.4, so we have not kept it up to date with the 3.5 stream.