So the one I was in had these massive rooms full of lead-acid batteries, and 3 or 4 multi-megawatt "generators" outside (located at various sides of the building, not all in one place).
They announced it over the PA because it was a planned transfer - and then all the "normal" lighting went off (think overhead fluorescents) at the same time the emergency lighting came on - the power to the servers and anything plugged into the racks didn't even flicker, of course.
There was a huge thunking sound from somewhere (I assume something disconnecting from grid power) and the muted roar of the generators starting. A few moments later, the main lights came back on.
It was an entirely different experience than a normal power failure, where everything goes quiet - you knew something happened, but other than that it all worked smoothly.
if you're into datacenter drama you may find this interesting. It's a livejournal of a datacenter in downtown New Orleans riding out Hurricane Katrina and all the crazyness they went through to keep things online in the aftermath. Comes complete with snipers, rolling barrels of fuel up parking garages, raiding snack machines, the whole enchilada.
Seriously, though, do tell us more.