I see that it's still available boxed, but does the boxed copy no longer work due to use of GFWL?

In any case, this again highlights the limitations of DRM / digital distribution model. The lack of a used market means that when games aren't available new, they aren't available at all. In the past, if a game stops being manufactured, you just buy a used copy and it's not that big of deal.

Edit: It sounds like the key from the boxed copy should be valid to activate the game on Steam. So I guess it's actually still possible to buy a Steam copy, you just have to buy the retail copy and use the key.