A number of users have asked about converting existing CVS and Subversion repositories to Team Foundation Server. While we have no plans to produce such, we had hoped third parties would. Today someone from Component Software posted an announcement of such a converter.
Here's the description from their web page.
CS-Converter is a powerful and reliable conversion tool, used to migrate from various legacy version control systems (such as RCS, CVS, Subversion and Visual SourceSafe) to Microsoft Team Foundation Version Control (TFVC) system.
Disclaimer: I don't have any relationship with the company, haven't tried the software, am not endorsing it, etc., etc.
[UPDATE 10/20/2009] With the Component Software product being discontinued, I wanted to mention that there is another company now with a tool to convert CVS and subversion to TFS called Timely Migration.