im riding fixed at the moment on a shimano deore hub.
all i did was remove the free wheel and weld the bearings up inside. give it a grind down and slot it back on. you have a fixed where you can change the chain line really easily. deore rear hubs are about £12 here, then get some old cassettes from your local bike shop and space out your cog.
i guess the difficult bit is building the hub onto a rim, but if you use an old rim and spokes then its good wheel building practice i rekon.

anyone else done this sort of thing?