I recently went down this road and i'd be happy to explain how to do it. Using all Sparkfun parts (yay!)
Arduino: http://www.sparkfun.com/commerce/produc ... cts_id=666
Motor driver: http://www.sparkfun.com/commerce/produc ... ts_id=8905
Motors: http://www.sparkfun.com/commerce/produc ... cts_id=319
Tank treads: http://www.sparkfun.com/commerce/produc ... cts_id=321
I controlled mine via Xbee but you could program it however you want.
The only hard part is wiring up the Motor driver. Picture from official sparkfun listing
ok, top/left side is all inputs. PWM+IN1+IN2 will drive one motor. Just make sure PWMs are connected to an arduino PWM line like Digital IO 10/11. IN1 & IN2 are normal digital IO lines. GND is the ground from your arduino. STBY is basically your on/off switch, i use Digital IO pin 13 because of the built in led.
Bottom/right side is mostly outputs. VCC&GND is from the arduino (power and ground). VMOT/GND is from your external power source to drive the motors, i'm using 2x 9v batteries in a series. O1/O2 goes to one motor.
Ok, on to using it. I'm going to use example pins, you are free to use whatever.
Motor #1, Direction pins 6 & 7 (6 on, 7 off = foward, switch for reverse)
Motor #2, Direction pins 8 & 9
Motor #1 Speed pin is 10
Motor #2 Speed pin is 11
So, set STBY pin 13 HIGH. Set pins 6 & 8 HIGH for forward motion. Finally set Speed pins both to 40 (mid-low speed). That's it.. you're moving.
Here's some fuzzy pictures of mine