Good idea, but then you have to find a bike with horizontal dropouts or track ends, which isn't that easy to do. You also have to go through the process of stripping the gears, cables, etc. AND purchasing and setting up the rear wheel, which may not even look good on the frame.

I'm surprised no one has mentioned the Kilo WT:

http://www.bikesdirect.com/products/.../kilott_wt.htm

It has rack mounts, a durable frame and it can fit crazy wide tires.