John Brost, one of the members of the MDP, recently posted to LUGNET a nice looking (and even nicer working) trike based around the NXT, and using the largest wheels ever produced by LEGO. It drives by remote control, and he has a video of it up on YouTube as well. Take a look:
NXT trike on NXTlog
I'm wondering if with putting the NXT back that far, and with the torque it can obviously put out, if it could do a wheelie. Perhaps with two small outrigger wheels to allow a rearward balance point. But it looks really good to my eye.