I'm used to my Macs being reliable; of going months between forced restarts. And yet, two weeks ago, I found myself restarting my unibody MacBook a couple times an hour. Apps would just lock up with the spinning disk cursor, and I'd go to another app and it would soon lock up to. I associated this with something fairly disk intensive, like doing a Spotlight search, and I found that if I avoided Spotlight things would be OK most days. I tried rebuilding the Spotlight database, reinstalling 10.6.0, upgrading to 10.6.2, etc., with no help.
Finally, under the working theory something was wrong with my 500 GB Western Digital Scorpio Blue drive, I ordered and installed a new 640 GB Western Digital Scorpio Blue drive. And this worked. I don't know if it was the hardware or the completely clean install or the act of reinserting the drive, but it worked and my Mac is back to its normal reliable self. And I've got more disk space than I know what to do with. I learned my lesson a few years back about waiting for a hard drive to fail; I no longer give them a second chance at catastrophe.
This might be the last mechanical hard drive I ever buy for a laptop. SSD capacities are overtaking mechanical, and I would certainly pay $300 for a 512 GB SSD of reasonable read and write throughput. Maybe this time next year.