I have a possibly dumb question . . . . .

Having worked with noisy PCs for years, from office environments to 
colocation facilities, I've been wondering: since all the components 
inside the box take power in such little sips -- 5 volts here, 12 there 
-- and since we know laptops can do it all on battery output, why do we 
still have big honking 200+watt power supplies that turn electricity 
into heat and noise, when it seems we could  do with a lot of it?

Is all the power to drive expansion busses and cards? if so, how much do 
we really need? I'd love to see one of these Spacewalker boxes designed 
to run fanless like the iMac or the departed Cube. I  realize their lack 
of expandability was key to the fanless design, but as in a laptop, 
couldn't one of these have all the goodies built-in?