Stays very cool and I never heard the fan roar, I actually never heard it unless I stick my ear on the computer.
Actually aluminium makes the laptop colder in general. The whole laptop becomes a heatsink. Since aluminium transmits heat well, the heat is dissipated all over the computer and doesn't stick at one spot, to a certain extent. You can still tell where the processor is (upper left corner) but it's more dissipated then on a plastic computer.
I guess I just said the computer wicks a large quantity of heat through the bottom so it should be hotter on your lap... But in my experience, laptops are mostly hot just where the cpu is (and sometimes a fan is stuck under there), most of the time, that's where it hits your lap (sitting down, I rarely put it flat on my lap) heating it only at that spot.
Also the battery and hdd are under the palm rest, so quite far away from the cpu and covered is by aluminium.
The ventilation is great on this thing, heat going straight up the screen rather than under or on the sides of the computer (I think the plastic MBs vented out the back). Stupid in both cases, hot air rises so a part of the heat is transferred back in the case and on the sides it ends up heating your hands and not getting pushed all that much away from the computer.
One things I had noticed with the powermac g4 is that the top of the computer (palm rests and keyboard) would get scorching hot under heavy use, mind you maybe that was just another hdd technology but the top never gets really hot. Maybe the chiclet keyboard has a bit to do with that, or just the ventilation.
Anyways, I rambled off quite a bit. All that to say that the computer doesn't get very warm on my lap even when running tons of stuff, including moderate snyth based garage band sessions.
On a side note, when it's really cold outside and you take your computer out of your bag, it's absolutely frozen for a while.
