We have just branched MOODLE_20_STABLE in git.
Since the 2.0 release last November, we at Moodle HQ only been working on the stable 2.0.x series, so it was a lot easier to stay on the master (HEAD in the old terminology) to reduce the amount of work needed to get new code into git. However, we are now starting work on 2.1 features, so the 2.0.x branch has been split away (at the end of last week).
For the next week or so at least the two branches will remain the same, so you have some time to deal with this, but if you are using git (or CVS for that matter) to maintain your stable sites, then you should make sure they are updating from the MOODLE_20_STABLE branch.
Even if you do accidentally update from the master/HEAD unstable version in future, a new feature in Moodle 2 (thanks David!) will actually warn you that you are about to upgrade to an unstable version and give you a chance to fix the problem.
Finally, what this means for developers is of course that you need to file three separate pull requests for most patches:
- 2.1 (master)
- 2.0.x (MOODLE_20_STABLE)
- 1.9.x (MOODLE_19_STABLE) (if applicable)
I'm still working on simplifying this process in the tracker.