Here's yet another project that covers the same ground from September 2010, for instance - http://pleasantsoftware.com/developer/3d/2010/09/26/keep-it-...
edit - another video of that one, http://www.youtube.com/watch?v=IaEkukmnmK8 and there is also this other thing from 2011, http://hydraraptor.blogspot.co.uk/2011/04/auto-bed-leveling....