I was one of the ones who had total system freezes every 5 - 6 days running on Tiger. After spending hours researching it, and going over Console logs, me and another Mac Tech determined it had to do with a buggy NVIDIA driver... which was causing WindowServer to crash.
Since upgrading to Leopard, and I'm assuming newer NVIDIA drivers that are included, I haven't had any problems. I would guess that this update bundles those drivers for Tiger.