Update 2-2-09: For now I abandoning the plans I had to create a relay based motor controller for the Mk.2. I received a Pololu motor controller for my birthday (thanks Winston!) and will be using that. Here is a picture of it:
Specs: 14amp continuous, 30amp peak, 5.5 to 16volts, separate PWM inputs for each motor.
Update 2-28-09: The motor controller was wired up last night at the Friday Night Build session, and so far has worked beautifully. I mounted it in an old PC power supply case with a 12v PC case fan for some additional cooling.
Motor control parts list:
-2ea PowerWheels type kiddie car motors, the ones I am using are 12vdc and have the rectangular adapter for a wheel. Some have a pentagon shaped adapter.
-Pololu Dual VNH2SP30 motor controller from here.
-misc. wire, connectors, terminal strips.
-spare darlington chips, see above. 
