From the old pages of Modern Mechanix "War Tank on One Wheel, Operated by One Man".
Housed inside the armored body, the operator will steer the single main wheel by means of two small auxiliary wheels at the rear. A turn of the handlebar lifts one stabilizing wheel and lowers the other, shifting the balance of the machine and turning it to one side or the other. An internal gear mechanism, operated by a motor inside the body, drives the wheel ahead at remarkable speed.
Link - via Make and Boing Boing
Previously on Neatorama: Monowheel | World's First Monowheel
http://carnalreason.org/images/misc/wheel.sp.gif