in addition to the above post, if someone simply wants a taste of fixed gear, or wants to do it on the cheap, using a normal freewheel road hub can work. Use a threadlocker on the threads, mount a bottom bracket lockring over the top of the sprocket if there's space, and tighten it on lots and lots using the rotafixa method:
http://204.73.203.34/fisso/eng/schpignone.htm
I've done a couple of fixed gear bikes this way, including two of my own, and the sprocket has held up to skidding, resisting and all that. As ever, run at least a front brake.
- Joel