A lot of interesting comments in here, but a few misconceptions that I noticed.
First, power when off is not necessarily zero, nor is power when asleep anywhere near the same as power when running.
I've checked myself using a Wattmeter, and older generation G4s use almost exactly the same amount of power when off as when asleep (4-10W, depending), so unless you turn off the power strip when it's shut down, there's no real advantage other than the fresh start to shutdown versus sleep.
The G5s on the other hand are better--they draw a few watts when asleep, but nearly zero when off, so they actually do benefit from being shut down.
In either case, startup is slower than wake from sleep, but restarts do tend to clear up funkyness (and clear the swapfiles) that happens after a while. I tend to shut my G5 down at least a couple times a week, particularly if I'll be away from it for a while. On the other hand, my G4 at work has been on (sleeping at night) for 4 months straight now, and it's running smoothly, so it's not necessary if your computer runs stably and is well maintained.
As for harddware wear and tear, cold-starting your computer and cold-starting your car are not really the same thing; the majority of wear on a car being started doesn't come from thermal expansion, it comes from the fact that the oil isn't where it's supposed to be or at its ideal temperature when the car first starts after being off for a long time.
Computers, obviously, have no oil, and the only moving parts are the fans and the hard drive platters. The wear on the motors of these parts at startup is probably greater than them just sitting there, but they're not lubricated the same way as a car engine and I wouldn't be so sure that starting them up once causes less wear than leaving the same thing on for a few hours--after all, startup just wears the motor (in theory) while being on wears the bearings and the motor, as well as any other components.
Shutting down solid-state chips involves ZERO perceptable wear--some chips, for example, can cycle themselves on and off rapidly to save power, and cutting power to a chip has no real effect on it--if anything, the longer the chip is hot (on, that is) the shorter it's lifespan, if you want to be technical.
Thermal expansion-contraction cycles could make a difference, but I'd say long term heat is probably more of a danger to most electronics.
It is true, in either case, that sleep and shutdown cause a similar amount of wear on the computer--the only real difference is that sleep keeps the RAM and a couple of subsystems that wait for wake (not including the processors) energized.
