See this post : Visual Studio 2005 SDK Version 2 has Shipped!!!!
For more information. There should also be updates in there for VSTS matching the RTM version. I hope they have done more than only upgrading, because some more detailed documentation would be very helpful.
Download it here: http://msdn.microsoft.com/vstudio/extend/ (and choose the april version)
You will need a VSIP login however, but registration is free.