Luckily for the dm Server team, we're not the first group in the company to move to Git. The Grails guys moved over a while back (http://github.com/grails/grails/tree/master) and haven't had any complaints from their community. To quote Graeme Rocher, the lead of the Grails project:

… we have definitely seen quite a few folks (I think around 40 so
far) forking the repository. Some of those have submitted pull
requests. The community reaction has been good, there has been no one
crying out for SVN to return which is a good sign.

Obviously we'll keep an eye out for problems, but I think we'll see the same thing Grails does.

I'll second the branching notion. We've always aimed to do developer branches (I come from a ClearCase background where that was the norm), but Subversion just kept letting us down. At first, it wasn't feasible with 1.4, then 1.5's support was half-hearted and caused us more grief than it helped. If you took a look at our kernel git repository since the move, we've really embraced branching and have dozens of them in only a few weeks. We're still feeling our way along with respect to reverse-merges, rebasing, etc. but I don't think any of our developers regret the change.

To be honest, I'm not sure there was a huge technical issue that led us to Git over something like Mercurial or any of the other DSCMs. In the end, Git had a very vibrant community and was easy to get started and educate our developers with. Those things more than anything technical sent us in that direction.

I'd caution against migrating the lot without some good feedback from end users, otherwise the likelihood of getting patches attached to JIRAs could decline. Naturally I think we might be expecting to see egit bundled with STS soon