Yeah it'd be up 24/7.
There could be 2 different servers.
(1) iBook G4 - Reliable but a tad slow. On 24/7.
(2) iMac Intel - Unreliable but fast. On when I don't turn my computer off.
Or hmm. I think I could do something fancy. I could have the server files be stored on my external drive, and whenever my iMac is on, just host the server there, and whenever it's off host it on the G4.
That'd mean 1 server, on practically forever and much faster whenever running on my iMac.
Though I wouldn't mind having a Minecraft client so I could test the server firsthand...