Floating discs on motorbikes are exactly as trundle describes. The disc itself 'floats' on the bobbins that connect it to the bell, spider or cage that is fixed to the hub, and the bobbins allow a small amount of lateral movement of the disc to ensure that the braking effort is applied equally by the pistons on either side of the calliper. A cheaper setup allows a single-piston calliper to apply a balanced pressure to the disc. The float is often quite noticeable, particularly on racing discs which will often rattle on the bobbins. An alternative way of achieving centralised pressure is a floating calliper, which is kind of similar to the way a single-pivot rim brake (should) avoid distorting a wheel under braking. The two-piece construction is about heat dissipation.
When I raced in karting (250 ICE, for those in the know), the discs floated on the axles using a connector not dissimilar to the Shimano centrelock system, and again the movement was distinct and sometimes noisy. Hence my assumption that they worked on a similar principle on bikes. But, looking closely, it appears that there's a compressible washer on the bobbins that allows a tiny amount of lateral movement on the spider and, as trundle says, it's not detectable by finger pressure - and floating disc of this type are available on both 6-bolt and centrelock mounts.
I have my doubts that this kind of float is particularly worthwhile on a bicycle (I think the hydraulic balance in the calliper will do all the self-centring needed), or that the heat dissipation properties of Icetech Freeza rotors are needed for most YACF-style riders. But they look nice, and often that's all that's needed!
