If the 'power brick' gets warm when it's plugged in even when the computer is off, it certainly is using power, but maybe not much.
I tend to switch off computers, and unplug them, because I'm concerned about damage from electrical storms. I've lost a computer that way before, so I'm probably over-cautious.
I'm not sure whether modern batteries suffer from being continually charged. It depends on the type of battery.
On the other hand, electronic equipment is more likely to fail during the application of power, because of stresses induced by transients. I'm just talking about normal transients, not 'surges' or 'spikes'.
So, there are 'good' arguments for both turning 'em off and leaving 'em on.
I might add that I tend to check the likelihood of overnight electrical storms, and then leave the computer running anyway. There's always something it could be doing.
http://superuser.com/questions/12358/is-it-better-to-use-laptop-on-battery-or-on-ac-power http://superuser.com/questions/12838/should-laptops-remain-plugged-in-when-their-battery-is-100-charged – random – 2010-02-15T09:19:07.320
I recently found this product for eliminating the waste of "standby" power. Might be worth checking it out: http://www.belkin.com/conserve. Cheers!
– macek – 2010-02-15T15:30:12.397