I just got a fat bike. I can't wait for winter, but I am using it in the dirt as well. I got the Motobecane FB-4, which was an amazing deal, even after putting $200 worth of upgrades into it (I instantly feel like I have to justify buying a Motobecane :-D).
It came with 72 TPI Vee Tire Mission tires which are close to Surly Larry tires in tread pattern. I started off at 10PSI and have worked my way down from there. I ended up at 5.5PSI (I measured after my ride) which was too low, I felt the tire hit the rim a few times and made me ride timidly. Just wondering what any fatbikers who are riding in the dirt are running your tires at. I was at 8.5PSI for last night's ride and it was pretty harsh, going for 7.5 for the next ride.