I've pulled together a short slide deck (well some consider 10 slides short) that provides an overview of Team Foundation Server 2010 Upgrade. This doesn't replace any of the official documentation and in fact be sure to download the latest Install Guide for Team Foundation Server after April 12th, 2010 for the lastest Upgrade instructions and checklist. (That is the best place to start for planning your upgrade.)
This deck is meant to provide an overview on Upgrade strategies, flow and some compatibility information. It is a work in progress and I will update periodically.
For more information on TFS upgrade please see this post: TFS Upgrade Guide.