We are in the process of testing the upgrade feature on TFS 2010, and in the next release you will be able to upgrade directly from either TFS 2005 or TFS 2008. Right now we are collecting real world customer databases to test against.
Specifically, we are looking for customers of TFS that are willing to share (via upload to a secure FTP site) a SQL backup copy of their databases with us here at Microsoft. We will then use these databases to test the TFS upgrade feature.
If you are interested in participating and can share your TFS database, please reply to me here, or email me directly at email@example.com.