I know Phil did a masterful job of explaining this on another thread, but this article from ARB has some good quick reference info. Perhaps it should be a "stickie"?
http://arbusa.wordpress.com/2011/10/26/the-lowdown-off-road-tire-pressures/
http://arbusa.wordpress.com/2011/10/26/the-lowdown-off-road-tire-pressures/