Naren Datha<, a dev who works on the WIT client, has written a post on how to use the new work item tracking custom controls that are introduced in SP1 beta (he previously mentioned some ideas for custom controls).  He provides an overview, details on how to create custom controls, some hints for troubleshooting, and a sample in a zip file attached to the post.  Check it out!

How to use Custom Controls in Work Item Form

tags: tfs, team foundation server, tfs api, work item tracking