There are three main sources I know personally for renting bikes for Burning Man. The Kiwanis Club of Reno Sparks, Da Rat and Black Rock Bicycles.
People love Kiwanis because it’s cheap — 30-50 dollars a bike — and at the end, the bike you ride goes to a kid in the local community. I just checked the Kiwanis site and they are no longer taking orders for burner bikes for 2011, but if you like the idea of supporting bikes for kids, keep the website bookmarked for next time.