pirred908: You're ignoring the other side of the argument. Leaving your computer on 24/7 will wear out the moving parts, but that's just the bearings in your fans and hard drives, which is a very small part of the machine. As a matter of fact, if something's going to break, you want it to be the fans or hard drive, because fans are the cheapest part of the machine, and hard drives are the only part that should have a 3-year warranty (OK, I'm exaggerating ... nobody wants their hard drive to fail). Turning the machine on and off, however, will cycle all the electrical components, causing them to heat and then cool, which of course causes them to expand and contract, which is the primary way for electronic components to wear out. Look at an incandescent light bulb, for example. If you leave a light on 4 hours a day, it will burn out much sooner than a bulb that is left on 24/7.
WackyDan: You must think you're pretty smart to make such an obnoxious condescending post like that, but unfortunately you don't know what you're talking about. You're grossly overestimating the amount of power that a machine uses. The average person is paying $0.07 per KWH for residential power. If your machine is actually using a full 250W (which is isn't, unless you've got an overclocked machine on a 500W PS with more than four 10K drives), that would come out to $0.42/day, or 12.60/month. The truth is that your average computer, monitor included, will use less than $10/month of electricity. I personally had an electric bill under $30/mo several times in a studio apartment when I wasn't using the air conditioning, and that was with two computers on 24/7, a TV and a full-sized refrigerator.