Looks like Microsoft's March slip-up that pinpointed the next milestone delivery date was correct. The Windows 7 Release Candidate (RC) -- the one and only public RC -- is set to be available on May 5.
I've been hearing Windows 7 Server, a k a Windows Server 2008 R2, also will be out in RC form the same day.
Still no absolute confirmation on what the build number will be for the RC. One thing is for sure: Windows 7, both client and server, are almost soup. (I'm still hearing this fall, most likely October, will be the official release date.)