I've had server 2k3 boxes that only get rebooted for updates, once a month or so, and have been fine for the 3ish years since installation. I figure it's due to start crashing in another year or two when the disk fails.
I had a freebsd box that had pretty stable reliable hardware, but the kernel threw a mountain of nasty clock spew because of a bug in the release... think it was back around freebsd 4.0 or 4.1. I had to downgrade back to the previous version for a couple weeks and wait for a patch, which wasn't fun.
The single biggest source of crashes I've seen in windows>=2000, linux, and bsd is hardware failure, particularly hard drive failures.