I sort of retracted this post please see:
Cheap BMX flip flop to fixed gear conversion
For an explanation.
If you have a typical BMX flip flop hub with freewheel threads on both sides I think you can convert it to fixed gear.
New! Surly Dingle Two-speed Fixed-gear Sprockets!
The treading works on the freewheel, not fixed, threading.
I bought the Harris Cyclery store brand hub, which is threaded fixed/freewheel but could have just found a used BMX wheel.