Jeff Beehler has just written a post about the newly-released December CTP of TFS. Jeff describes some of what changed, but probably the most important thing is that it is NOT supported under the go-live license. There will not be any migration scripts to move to it or from it.
The reason for that is that this release hasn't been through much formal testing. Sure, we upgraded our dogfood system with it, so we think it works well enough. But the goal here was to get a drop out for folks to test out all of the changes we've made, though not in production use.
If you do try it out and find a bug, or if you find a bug in beta 3, please file it through the MSDN Product Feedback Center. All bugs filed there get reviewed and assigned to the appropriate dev, as appropriate. The bugs from PFC are automatically imported into TFS Work Item Tracking in our dogfood system.