the shimano spd system has no float. once you rotate your foot, you're out of the pedal.
you can adjust the tension, but that's not the same as float.
the look keo system allows you to pick cleats with different float options, so you can have a little side to side "play" in your foot before it releases....only this takes a dedicated road shoe, and those have slippery soles and are hard to walk around in.
the crank bros system acts the same way...15* float or 20* float, depending on which shoe you have the cleat on...
see: http://www.crankbrothers.com/tech_mallet.php

i've heard that crank bros isn't ideal for fixed though (i don't know why), but i use crank bros on my crosscheck (simply for the ease of having a walk-around-in shoe) and every mountain bike i've ever owned (for the 4 way enter/exit and mud shedding) and love them.
but for a dedicated road bike, look is the way to go...the bigger pedal surface cuts down on hot spots and has a greater power transfer over smaller cleats like spd and crank bros.