Well this thread finally got me motivated to "fire-up" one of my eMac's (I haven't started this one up for probably 2+ years)!
This eMac is running OS 10.4.11...within about 5-10 seconds I was signed into my Airport wireless network (I got a new router within the past 2 years, and needed to enter the password).
I just clicked on the airport icon in the menu bar, chose the network I wanted to join, and entered the password...BAMM...I'm in!
Apple really makes these sort of things pretty darn simple...so if everything was installed correctly, and if the airport card is functioning...hard to say what's wrong...other than the airport card is no good (which can happen).
- Nick
p.s. If you're running OS 10.4...are you updated to OS 10.4.11?