We've worked hard to make sure TFS 2010 is usable from all clients that can access TFS 2005 and 2008 in order to make your upgrade to TFS 2010 easier.  Unfortunately, the changes in 2010 are significant enough it will require patches/updates to older clients to work fully.  The follow post explains the compatibility philosophy and details the issues you might run into.

http://blogs.msdn.com/teams_wit_tools/archive/2009/10/19/compatibility-matrix-for-2010-beta-2-team-foundation-server-to-team-explorer-2008-and-2005.aspx

The patch for VS 2008 Team Explorer clients is available now here: http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=cf13ea45-d17b-4edc-8e6c-6c5b208ec54d

An update for the MSSCCI provider will be available in mid-November and an update for VS 2005 clients will be available in the 2010 launch timeframe.

Brian