Just looked this up for a customer, so I figured it was worth sharing more broadly. If you're planning to migrate a Visual SourceSafe 6.0 database to Team Foundation Server, you'll want to check out the following articles:

http://blogs.msdn.com/akashmaheshwari/articles/474262.aspx

http://blogs.msdn.com/akashmaheshwari/articles/474260.aspx