For many months, I chased after the cause of my computer, a late-2006 MacBook with Snow Leopard and all available updates installed, locking up, forcing me to restart. It locked with a spinning cursor and didn't respond to ctrl-option-esc, or to anything other than a power switch forced restart.
I have lately discovered that the freezes were caused by running out of memory. Using FreeMem Pro, I'm able to monitor memory usage, and to prevent the locks by freeing up memory when it grows short.
The problem seems to be related to gradual growth in the size of wired memory, which starts out at about 1/2 GB and grows to 1.5 GB or more. (This model is limited to 2GB of physical RAM.)
In turn, the growth of wired memory use appears to be related to Safari. Quitting Safari releases a big chunk of memory to free memory. Relaunching Safari then begins chewing up wired memory once more. I've scoured the Apple User Forums and other such resources where I found similar reports to mine but no worthy answers to WHY it occurs.