Andy,
The hold up is nasty things like Eclipse bug 52795 that broke resource lookups and XSL transforms along with a few otheres. With each release of a milestone build, we have to migrate all of our changes from our 2.x product in such a way that we don’t break the many 3.0-specific code changes we had to put in place due to all the API changes that showed up beginning in M5. It’s simply very time-consuming to port, retest, and work around bugs for the entire product for every new milestone. Remember, unlike other Eclipse add-ons, this isn’t ‘a plug-in’; it’s ‘a product’ that includes over 60 plugins. 🙂
However, we’ve been doing a lot of that work in parallel with the 2.7 final testing so 3.7 should only lag by a few days if no additional showstoppers are found.