I've had my 2.16GHz 17" MacBook Pro since they first came out, and I only recently began to have this problem. I would routinely have uptimes of WEEKS (and only then because I shut it down to take it somewhere), and my G5 cranks out uptimes of months, and only then because of security updates.
Then suddenly it began freezing around Halloween 2007 culminating in a total hard drive failure. Luckily I began backing it up when it started crashing a lot before the drive totally failed. I was a little ticked off, but upgraded the stock Seagate 7200rpm 100GB for a Hitachi 7200rpm 200GB (because I heard about the problems with Seagate) and got Leopard installed while I was at it.
Everything ran fine until around Christmas, where it began freezing TOTALLY a few times a week (sometimes a few times a day). I tried everything you all talked about here, such as Disk Utility, Diagnostic, Disk Warrior, etc. but not TechTool Pro (which sounds like it won't tell me anything either). I tried zapping the PRAM, trashing the OS cache, etc. too.
Until I read this forum, I thought I might be overheating it by running it hard. I run processor intensive apps, and run BOINC when I'm not, so I'm running around 100% processor 24/7/365. The processor temp usually hangs out around 80C (the hard drive is at about 40C). I didn't buy the 17" primarily for portability, I bought it for the screen and the performance to run the apps I need, so I'm not very receptive to criticism that this is happening because I'm running it too hard. Besides, my G5 and G3 (although desktops) ran for years with no problems. After reading this forum, I'm pretty sure it's not a heat issue anyway.
One final note: sometimes I get a little warning that it's going to freeze because some graphics artifacts appear on the screen. For example, sometimes I get a transparent light blue swoosh across part of the screen, as if it were a layer of the genie effect of putting something into or out of the dock. Other times I get something like a line of TV static a few pixels wide that runs horizontally across some random part of the screen. Anyone else had that?