When we were planning the requirements for TFS 2010, we decided to take a dependency on Office 2007 and later.  This enabled us to make some significant improvements to the TFS integration due to extensibility improvements in Office 2007.  However, I've seen a few customers who are stuck on Office 2003 and have been disappointed to see that Excel and Project integration with TFS stops working when they install VS/Team Explorer 2010.

It is possible to get it working again (with the 2008 client).  John explains how here: http://blogs.msdn.com/team_foundation/archive/2010/04/24/vs-2010-and-tfs-with-office-2003.aspx