Andrew,
I have used the Arduino to drive a stepper driver (Big Easy Driver) for a camera tracker. It was very easy and I am sure that you will be able to modify the program with a few switches and conditional loops to use it for a motorised focusser.
Look at the following links:
http://starsinphotos.wordpress.com/2...amera-tracker/
http://littlebirdelectronics.com/pro...-easy-driver-2