There have been quite a few powered unicycles that I've seen. They're usually gryo stabilized and supposedly easy to use since it balances on it's own. Honestly I'd prefer one of the uni-wheel cycles were you sit inside the wheel.
I've seen one built out of an old back wheel off a larger tractor, which wouldn't be hard to get ahold of around here. It's at 0:30 in this next video.
The hardest part is steering. If you want a totally true monowheel, then it is usually done with either a vertical fin on the back like a boat or plane, or with leaning stabilized by the gyro. If you're more willing to compromise it can be done with two small wheels on struts,like normal bicycle training wheels, that hang out and allow lean turning. One of the easiest solutions it two make in not one wheel, but actually two narrower wheels right next to each other, and thus allowing it to steer but changing the speed of each wheel.
When looking for the tractor wheel cycle I also found this thing that looks like a really old and up-scaled version of the uni-skateboard. It appears that it might be semi-amphibious, and if not, I could make a modified design that is. This would benefit greatly fro the side-by-side wheel design. It's hard to tell the depth of the picture and thus the width of the vehicle, but it might use that wheel arrangement anyway. I don't see any other method of steering. It would also need sliding weights on the sides that move automatically to compensate for the weight of the driver, and load, and the torque of the wheels. Sacrificial skid plates on the ends might not be a bad idea either.
Yes, I've put thought in to the engineering and building of one of these, why do ask?
Edit: Turns out the last one had a video of it and a couple similar prototypes in operation. The one pictured is actually a two-seater and had a pretty good speed capability.
Video:
http://www.me.berkeley.edu/one_wheel_vehicle/Movie.htmOriginal page I found:
http://grassrootsmotorsports.com/forum/sprockets/not-a-monocycle-a-one-wheeler/38073/page1/