Good to know. Intuition suggests that yeast cells actively producing CO2 would be more bouyant than yeast cells which are not producing CO2. Similarly, ale yeast (at warmer temps) are more metabolically active and produce more CO2 in a shorter period of time, so it stands to reason that, relative to lager yeast, more foaming would occur, more headspace would be required (unless antifoam agents are used), and that more ale yeast would be found in the krausen.
The fermentation temperature has a huge effect on yeast cell metabolism and it is the sugar metabolism rate (and CO2 production rate) that dictates the relative amount and position of the yeast cells within the fermentor. Since the metabolism is slower for lager yeast, you don't need as much headspace for your fermentor.
Purists will still say that lager yeasts (S. pastorianus, nee S. carlsbergensis) can fully metabolize melbiose, raffinose, etc. and are genetically distinct from ale yeast (S. cervesiae). Other lager yeast strains include S. bayanus and S. uvarum. Anyone who has watched the hypnotic effects of active fermentation (of any yeast type) in a clear fermentor will agree that there's a lot of movement during fermentation, so it's probably time to stop using the inaccurate terms "top-fermenting" and "bottom-fermenting."