Since September, Pacemaker has started using Git for the 1.1 and devel trees.
There were some minor technical advantages over Mercurial (which I still personally prefer), but mostly the decision was driven by the pain associated with switching between SCMs multiple times a day.
The majority of development now happens on GitHub, which has some great features for reviewing patches and general collaboration.
The Pacemaker tree is also periodically sync’d to the ClusterLabs server in case GitHub is unavailable for any reason.
For those new to Git, GitHub has many tips for setting up Git, creating a local copy of the Pacemaker repo to work in, submitting your changes upstream (we use the Fork + Pull Model), and other assorted resources.
Be sure to configure email and user information so you get credit for your hard work too!