On my 33x10.50 tires, I'll run anywhere from 20-50 psi on the road (the narrow BFGs are rated to 50 psi, the wider ones only 35 psi), but usually 30-40 for long highway trips. Off road anywhere from 7-15 psi, depending on terrain.

Some tips on determining proper inflation pressures:
http://www.4crawler.com/Diesel/Tires.shtml#ProperInflation