Given the age of the unit, is it possible that your unit doesn't support 11g and that's what the hotel is using? I'd have to go back and double check the specs of your unit, and I don't know which PBG4 you have, so I'm just thinking that those models probably straddled the line when 11g took over from 11b, and Apple is rarely one of the first to adopt new technologies, so there's often an additional bit of lag before support catches up in the Apple world.
I have an odd airport issue. My Powerbook G4 will not detect the wireless network at the hotel I am currently staying in. I have taken it and put it next to the router and nothing. I have also had the hotel reset the router with no luck. A co-worker with a newer Macbook has no problem. Oddly, when I go anywhere else in town that has wifi I have no problem. Can anyone shed any light? I am using 10.4.11 on it.