I just found out about SlimDX, and I think this is perhaps the best news I’ve gotten in a long time. DirectX from Managed code was a god-awful mess, what with MDX 1.1 being frozen and MDX 2.0 thrown out the window. Xna was never really the successor to MDX because it was meant as a game development API and not really a graphics API exposing core GPU functionality to programmers.
Here are some of the reasons a Managed DirectX wins out over Xna. Of course one of my pet peeves is video support for Xna. There are two good threads you can read through about this, here and here. With DirectX 10 support (albeit alpha), this is definitely a move in the right direction.