I hate to advocate turning off your mac, rather than sleeping it, but it does use less power when asleep. Yes, I know its only a couple of watts (I think i read somewhere that a FlatScreen iMac uses ~5W when asleep, ~3 when 'off'), but over the course of the year, it all adds up. Why should this bother you? I dunno, most likely it doesnt. But, if as everyone leaves their computers on sleep, instead of 'off', or even all-the-way-off (ie plug & batteries removed) then the amount of energy used up is far greater...
And before I get eaten, I agree with most of the plus sides to sleeping over shutting down, and tend to leave my powerbook alseep unless im gonna not be using it for more than a couple of days (so its on/asleep most of the time). Im just pointing out an argument, possibly the biggest argument, for shutting down your computer. And turning your telly off at the mains. And all the rest of it, for every appliance you have that goes into standbye instead of all-the-way-off....
Yeesh, i sound like a parent - "Turn off the light when you leave a room!"
Other thing - the guy I live next to at uni goes absolutely nuts when I show him my uptime and how many apps/windows ive got open - his advice is that you should turn your computer all-the-way-off and leave it there for at least 24 hours at least once a week, to give the processor a chance to cool down to room temperature and have a rest. He seems to think that not doing so can shorten the life of a processor. BUT, ive seen nothing to back this up, and to the best of my knowledge, its not happened to any computer Ive ever seen. He uses windows/intel anyway, so isnt used to the joys of MacOSX, though methinks he plans to get a powerbook....
That sure was quite a ramble... Please dont eat me too much!