Simply by using Pulse width modulation IE varing the duty cycle of the voltage to the motor will do the same. Kind of like how a DigiQ-iRacer-Booster car works.
Oh here is the guys site:
http://www.age.jp/%7Eex2/bit.html
Its a bit of the way down and remember to use altavista translator since its in Japanese.
Bet withing a year or so the cars will be smaller and full speed steering variable!