I've built a couple wheels for fixie and track riders on Powertap hubs. (These, by the way, are hubs that have pressure transducers inside that enable you to measure wattage output, which allows you to track your training and to test your output at different gearing, positions, etc. Very valuable if you're racing, interesting to know in a self-improvement perspective otherwise.) There are two ways to modify Powertaps for track -- one is to use the Shimano freehub to fixed conversion from Surly, and the second is to buy an actual re-machined Powertap hub. The former you can use with the new Powertap SL, and the latter is available (so far) only for the older all-aluminum Powertap. The primary advantage of the SL, in my mind, is that you can have a non-contact sensor that picks up the signal from the transducers through the carbon shell, rather than needing a continuous electrical contact through the side of the hub. It just makes the hub more reliable. That being said, the SL had some problems out of the box, which Powertap has supposedly dealt with at this point. Be sure your hub is a revised version if you go to an SL. The remachined one is completely adapted to track dimensions, including conversion from 130 to 120 mm.
They lace up just like any hub and you can read power, cadence, speed, heartrate, etc. all on one monitor on the handlebars. As an alternative, you can get an Ergomo for the same price, which I've found is a bit more accurate and slightly more reliable. It has the sensors incorporated into a Dura Ace 7700 series splined bottom bracket, so you can use current DA track cranks or 9-speed road cranks of any version. It's very elegant and about the same price as the SL. Then there's the SRM, which is outlandishly expensive but is the gold standard and works better than any of the others with transducers mounted in the right crank arm. Get a Dura Ace 10 version and use a DA-10 road crank as in a conversion rig, or there's a dedicated track version (which, however, is nowhere near as stiff). You can google for manufacturers and retailers for all of these systems.
Fun toys, and very valuable if you are racing seriously, but they all need care and are not exactly typical fixie gear. Oh, and you need a reasonably competent Windows computer with Windows XP to download data from all of them and analyze it (if you don't do this step, you miss the whole point of the units).