Checking number of reallocated sectors of HDD through Disk Utility, Mac OS X
Reallocated sectors frequently (but not always) appear when a hard drive stars to fail. Just a few back-of-the-envelope calculations here:

If you bought your computer in the beginning of 2008 and used it 24 hours a day, you'd have used about 32,000 hours of hard drive time. I don't know what model of hard drive your computer has, but the mean time between failures for not-pariticularly-impressive hard drives from that era is about 300,000 hours. So in the heaviest-use scenario, you've used your hard drive for about 10% of its expected life.

I don't think there's anything strange about a lack of failed sectors at this point.