The TFS technical writers have been busy putting together documentation about the activities that ship with Team Foundation Build 2010 and this is now available:

http://msdn.microsoft.com/en-us/library/gg265783.aspx

On this page you’ll find both a goal-oriented listing (i.e. Get the values of environment variables) as well as an alphabetical reference by activity name. You can provide feedback through the Community Content section at the bottom of each page or general feedback about the format can be provided on Andy Lewis’ blog.