I was good friends with a hardware engineer who was part of the development team for a variety of Maxtor drives. He explained that all hard drives must fail, it's not a question of "if", only a matter of "when". Reason being part of their design uses an extremely hard metal pivot bearing that slowly wears out due to the high rotational speed (7200/10000 rpm) . Because of the confined sealed space and the high generated temperatures engineers can't use lubricants as they would simply vaporize and possibly contaminate the drive platter(s). This is why all hard drives are rated in MTBF (Mean Time Before Failure), meaning they know it's going to fail and xxx hours is about the time, on average, it should last.
With that said there's a tradeoff in turning your computer on/off vs. leaving it on all the time. If you turn it on/off the main "shock" to the system is not simply the mechanics of the hard drive but the electronics in the motherboard, power supply, CPU, etc. In contrast, if you leave it on all the time you prevent the "shock" to the system but you add wear to the hard drive (and, incidentally, all your cooling fans) by using up hours of its MTBF. Maybe, as some have suggested, powering down the hard drive is an alternate approach. Personally, because hard drives are so cheap, I keep my systems well backed up and turned on all the time with, of course, monitors turned off when I'm not using them (as even LCD panels have backlights that burn out over time).
BTW, the moral of this story, whether you choose to always leave your PC on or not, is to back up religiously. Otherwise, if you put it off, if you never quite get around to doing it, I can assure you that when your hard drive craps out and your lose all your data, software configurations, etc. you will get religion.
