Philphine said:
it just occurred to me. if your putting force on it backward to stop, what are the chances that you just unscrew the rear sprocket and then can't stop? or does the rear sprocket attatch some other way?
If you get a true fixed-gear hub, it will have threads for the cog, and smaller reverse threads for a lockring. That way it will never unscrew. Some people go the cheap route (probably what I would do) and thread a track cog on a regular freewheel hub, and then use a lockring from a 3-piece bottom bracket with some loc-tite (it's the same threads) to keep the cog on. Most "flip-flop" hubs will have straight threads for a freewheel on one side, and track threads on the other, so you can turn the wheel around and use a freewheel if you want. They do make hubs that are free on both sides, as well as fixed on both sides.
Notice the stepped threads on the left side, and straight on the other:
(EDIT) Also wanted to add, just because you see threads on both sides of a hub doesn't mean it's for fixed gear riding. Newer BMX hubs have standard English threads on one side, and smaller Metric threads on the other, to use freewheels smaller than 14T. They also make "southpaw" BMX rear hubs, that are made for left-hand-drive bikes and have regular threads on the right, for regular bikes.
Plain freewheel threads on both sides:
English on one side, smaller metric on the other:
They also make Tandem hubs that have threads on the left side, these are made for old Drum or Band brakes, but can be used as a flip-flop if you're desperate. Some of these have disk rotor adapters that thread on the left hand side, too.
Tandem hub:
Lots of choices, make sure you know what you're getting!