Rolling code tx and learning

Hello all

I saw sparkfun's RF Link Transmitters and would like to make up a rolling code tx. I used to have a homelink system in one of my cars and would like to make something similiar. Has anyone done this or does anyone have some good info on where to start? My google ninja search skills are failing me.

Thanks in advanced!! Joe

http://www.atmel.com/dyn/resources/prod_documents/doc2600.pdf

It's not for the atmega168 but two other chips. I don't think you get the code unless you buy the dev kit.

Thanks!