Just recently, a new tool called Scrat was released. It enables migrating artifacts from HP Quality Center to TFS. It is the most comprehensive tool I know of, handling Requirements, Bugs, Test Cases, Attachments, History and Links between items). I haven’t had a chance to try it myself but I’ve known and worked with one of the authors (Shai Raiten) for quite some time and always been impressed with his work.
Your alternative to this would be either using HP’s or Juvander’s tools to migrate Bugs and Requirements and then using our Excel based test case import tool and I still don’t think you’d end up with everything Scrat can migrate.
If you are looking at adopting TFS and need to look at moving some or all of your QC assets into TFS, Scrat is definitely worth a look.
Here’s an url where you can learn more: http://www.sela.co.il/scrat/main2.html