Inspired by John Arne's post, I got a Redcat Rockslide, which is totally cool and a great robotics platform. As you can see, it's really easy to add a mounting platform for electronics (that's APM with GPS and Xbee mounted above) and there's LOADS of room. You could even have it carry a laptop if you wanted. BTW, that's the 1/8 size Redcat shown above, which is $236. I think the 1/10 size Redcat would probably be just as good for most uses, and it's just $159. [Update: the 1:10 isn't big enough. See this comparison]
I've registered the ArduRover name, and am now just looking for the best starting point to modify the APM code for ground rovers. Since James Goppert is part of the APM dev team, we'll probably start with his rover code, which you can see at work here. But if others have code or sensor additions to the basic APM set that would be good to add to a rover, this is the place to discuss and share.
Ideally, we'll get this done in time for the Sparkfun AVC contest, on the hope that APM can have a decent chance of placing in all of the categories--ground, plane and quad. Which would be awesome.