This question came up a few times lately in the Sculpfun forums, and I had this project on my list since February. Now I was able to finalize it and write some guideline about it. The article shows how to automate the z-axis, which can be used to have automated focussing to the workpiece or incremental z-adjustment during deeper cuts. It’s nothing you really need in daily business, I guess, but still a nice thing to try out! 
Full article: Automatic z-axis / focusing - Diode Laser Wiki
If you have anything to add to this article (better method? different commands?), just let me know!