New housemate has an old 1st-gen iBook 466mhz (blue!) running OS X 10.2.
Our house is sharing a Qwest DSL line using one of their integrated routers.
It wasn't able to get on our house wireless (since the house used WPA2 and WPA support is for OS X 10.3 and later).
Even after a 10.3 upgrade, even after switching to 40-bit WEP, and even after shutting off encryption entirely, the symptom was the same - it could SEE the wifi network, it could tell a password was requiered (when encryption was on), but would nearly immediately kick out an error of 'There was an error joining the AirPort network "foo"'
Even booting an Ubuntu PowerPC liveCD showed (from the command line) thaat the card would associate with the AP, then disconnect 1-2 seconds later.
Solution: WiFi router was set up to use Wireless 'G' and 'B' both. Disabling 'G' and setting the router back to the older 'B' only caused the Mac to be able to sign on perfectly.