Team Foundation Server Release Candidate will be available for download by MSDN Subscribers later today. Rob has put together some detailed notes here on upgrading to the RC (as well as some incentives for feedback):

http://blogs.msdn.com/robcaron/archive/2006/02/06/526353.aspx

Also of note, if you're a partner, the TFS Extensibility Kit which is now part of the Visual Studio SDK will not be updated till the VS SDK CTP in March. The most recent one would be from the Dec CTP which you can download from the VSIP site:

http://msdn.microsoft.com/vstudio/extend/customize/