According to the documentation on this site in Tutorial #6 , it seems that you can create control applications if your robot doesn't run the .NET framework (likely if you decide to roll your own). From the tutorial, it looks like it would be possible to write an interface to control pretty much anything you can come up with, though I admit I didn't give it an exhaustive consideration.
That being said, it does seem to support a wide array of robots according to Wikipedia and several of those models aren't exactly "high-end". You may find something up your alley, and it would undoubtedly be easier to start out with, say, a Parallax BOE-Bot than building something from scratch and creating all the interfaces.
There is, apparently, a simulated robot environment as well. According to the Wikipedia article linked above, you can, "simulate the behavior of robots in a virtual world using NVIDIA PhysX technology (3D engine originally written by Ageia) that includes advanced physics." Microsoft makes extensive reference to the Ageia physics engine on their site as well. If you have an idea you want to try out, this might be a free way of doing so. I haven't really looked at this environment, though, so anything other than what I quoted from the sources is speculation here.
I have not used the MS Robotics environment at all, so all of this comes from the documentation.
