This bike has a flip-flop hub with a Shimano 18t freewheel on one side and a 17t fixed cog on the other. I bought the wheels online from a bike shop in Wisconsin. They are 36 spoke wheels and have the cheap Suzue Jr. hubs. You hear a lot of internet noise about how terrible these hubs are but I have kept them adjusted and lubed and have ridden them for over a year (since Sept. '05) without any problems. I am 6'2" and weigh 260 pounds.
This bike is fixed only with a 17t cog on one side and a 19t on the other. I built these wheels myself using IRO hubs, Sun CR18, 36 hole rims and Sapim 14g spokes. They have remained true for ~700 miles now and I don't anticipate any trouble. It weighs 23 pounds as you see it here.
You can see more photos of these two bikes in the links in my signature (3029, 3502). I am in the process of building a Fixed/Singlespeed rigid frame Mountain Bike... hope to have it done by the first of the year.