Some one posted this over on MORC, and I thought it was very interesting:

http://www.bicicletta.co.za/Downloadabl ... trated.pdf

It is a study of the different rolling resistances of different tread patterns, widths, and tire pressure. The report focuses on mountain tires but does a great job of talking through some counter-intuitive information. It talks about differences between gravel, grass and road surfaces.

Geek it up - I am now reading scientific studies on rolling resistance...

http://www.terrymorse.com/bike/rolres.html

This one is the same sort of thing done for road (clincher) tires - if you click the refs, you can view tubies too (note that they do not inflate the tubies very hard - I run some Tufo's better than 160 at times)