Stolen right from their blog: "After working with several customers on their migrations from ClearCase to TFS, it was evident that one of the big challenges of migrating between such vastly different version control systems was determining an appropriate branching structure. This was the result of both the varied branching models of TFS and ClearCase, as well as the best practices encouraged and best adapted to each system. Below are some high level explanations of the branching models of each system, how the aforementioned migration tool migrates branches, and recommendations on how to choose the correct set of branches to migrate from ClearCase to TFS."
This is the long awaited tool you've been waiting for ClearCase users! Check Matt's blog for more info: http://blogs.msdn.com/tfs_migration/
You can download the tool from the Team Foundation Server Migration and Integration Solutions: http://msdn2.microsoft.com/en-us/teamsystem/bb840033.aspx
And finally, since they often go hand-in-hand, don't forget there is a Converter from ClearQuest to TFS as well. Here is more info on how to use it: http://msdn2.microsoft.com/en-us/library/aa337614(VS.80).aspx