Microsoft is said to have started the development of the Windows 7 Release Candidate 1.
The Release Candidate 1 of Windows 7 is said be under construction already. At least the Microsoft expert Chris Holmes says so under reference to unmentioned sources.
This thesis is based on the build tag that has become available recently. The build concerned is tagged with 7048.winmain_win7rc1.090211-1625 and thus bears the mark usually used for Release Candidates.
Holmes expects that the Release Candidate will become available within the next four to eight weeks. So it seems like the works on the Vista successor is making progress.