I have a back office box running FreeBSD that does all kinds of neat things like file & web sharing services, firewall, RSYNC backups and a couple of PostgreSQL databases (for the web services). It was running for over 37 months before last weekend, when I patched it and rebooted.
The Mac Pro I keep under the desk went nearly 3 months without a reboot until Apple decided it was time to start patching on a regular basis again. Outside of patch rebotts, though, I can say that I haven't had to reboot the machine otherwise since it was delivered to me 23 months ago.
The frankenbox that sits next to the Mac Pro gets shut down every night. It's running Vista Premium right now (with the free upgrade to Windows 7 whenever it comes out). Since this machine is primarily a gaming and experimentation rig, the only time it doesn't get rebooted on a nightly basis is when I am burning in new hardware or testing new OC settings.
I will have to go look at the Mythbox I have out in the living room, but I know it gets rebooted quite often, at least whenever the power strip gets turned off whenever my wife or I decide to vacuum.