The Visual Studio 2010 Beta 1 SDK documentation just got released!  You can access it from here:

http://msdn.microsoft.com/en-us/library/bb166441(VS.100).aspx

 

There's still a lot more to cover but this set of docs will get you started with what's changed in VS 2010.  The team will also continue to blog on this site and our own personal blogs about other cool extensibility features so keep an eye out for that.