So far I've:
- Set up a cross-compiler toolchain to compile code for the Pi on x86 boxes I have on hand.
- Got the BCM2835 peripheral datasheet at ready reference
- Started coding a driver by examining existing Linux I2C drivers and using parts of a couple of them as templates. Having and having read and used the "Linux Device Drivers" book should help ;)
- Found two or three people who already have a Raspberry Pi who are willing and able to test driver code.