By itself, Windows 2000 and Windows2003 can stay up for long periods of time. I have one server that has been up since 2004-04-29, but we tend to patch a lot. If you have some application on the server that has a memory leak, then you may have to reboot on some sort of schedule to clear the memory usage. Clear as mud?
My take is that it's an urban myth. MCrowley is correct when he states that it depends if you have any apps with a memory leak. If so, then you're memory usage will grow over time, and you will eventually start paging. If this happens, then you have to reboot.
If you don't have any memory leaks in any of the apps, which is easily tracked by monitoring the memory usage, then you shouldn't have to reboot for a long long time. I've easily gone three months without rebooting on Windows 2000 and I've gone over six months on Windows 2003. The difference between them is that 2003 requires less reboots when applying patches or making system changes.