it seems we're not alone with this sort of issue. There are similar postings at the following thread:
http://discussions.apple.com/message.jspa?messageID=11631251#11631251
so far my lock ups have only occurred when the computer is idle, never when I'm working on it. So if I leave the computer and it only has one or two things running (eg say a web browser and chat window) then most likely when I come back it will be frozen.
If, however, I run many apps, like apache, and mysql, and rails, and leave those running, then it makes it through the night and is still running just fine when I come back in the morning.
I was hoping that it might be related to the energy saver setting 'Put the hard disk(s) to sleep when possible', but I turned that off earlier tonight and I experience a lockup since then.
I have two USB 2.0 external disks and a USB 2.0 flash drive plugged in. I will try unplugging those and see if the computer still locks up.
Oh, and yesterday, I did get a lockup after having run Disk Utility by booting off of the install disk. Disk Utility reported all was well...then i just left the computer for a few minutes and came back and it was locked. So that tells me that my problems isn't a software conflict or anything like that on my install of Snow Leopard. But it could be a firmware issue or an issue with Snow Leopard.
Anyone have any theories?
I spoke to Apple yesterday and the only thing they were able to suggest was to take it for service. I'm not convinced that its an isolated issue with my computer though.