[apalrd]

As long as you feed them a good PWM signal at 5v levels, they will be very happy and run your motors.

If you miss an update, they will stop your motors.

If you are using a Jaguar:
Pulse between .8 and 2.2 ms long(I think this is correct)
Center is at 1.5 ms long
Update at least every 20 ms
Update no more then every 5 ms

Pulse between 1 and 2 ms
Center is 1.5 ms

Update at least every 50 ms ish - I dont know this exact time, sooner is better
Update no more then every 10 ms

All times are in milliseconds.

Since you are on Arduino and using unsigned integers, you should probably store the speed value as a uint8 and then scale it to a uint16 (microseconds) for output.

Edit: Squirrel beat me to it.