I expect we'll see more of this kind of stuff, especially something from GitHub at somepoint. Now that Microsoft is going full-out with Git support in VS, you can bet there is going to be some top-notch support. I love Git Extensions, and SourceTree looks great, but the appeal of being able to have VS with work item, build, and Git all together is pretty strong.
GitHub for Windows is not really comparable to Git Extensions or other full Git tools. It seems like it's point is just to help nudge Windows users onto GitHub. I imagine GitHub will revamp this position, especially with respect to their bug tracking integration and so on.
