After many hours trying to get an old iBook running 10.3.9 to connect to a D-Link DIR-655 router in mixed n,g,b mode, and failing, I finally solved it by changing the mode to g,b only.
I had updated the Airport software to 4.2, checked the firmware version of the airport card in system profiler, deleted all old wifi network keys, trashed the airport.plist prefs, etc, etc. I also tried both Auto WPA/WPA2 with TKIP and AES authentication and plain old WPA with just TKIP. Same results. Nothing worked except changing the speed mode.
So I've sacrificed the faster n mode on my network for the sake of this old iBook.
My question is who is to blame here? Is it an Apple software problem or a dlink firmware problem?
(The mixed n,g,b mode should automatically adjust, right?)
note: this also posted in Apple Discussions:
http://discussions.apple.com/thread.jspa?messageID=11553430