The AirPort card in my Titanium 15" PowerBook no longer would connect to my home wireless network, using a Linksys 802.11b router. I swapped the AirPort card for one that I knew worked. Still it would not connect, though the PowerBook had connected in the past with earlier versions of OS X (can't recall how far back before it stopped working; I am running 10.4.3 now). Also, I did the latest AirPort card update, to no avail.
Then, thinking it might be the router, I replaced it with a Linksys Wireless G (802.11g) router, and it still did not work. A wireless-connected HP printer worked with both routers, though. So, I did some fancy Googling and located the Sonnet Aria extreme Network Adapter (the model is G54-CB), which is claimed to work in 15" Titanium PowerBooks. I inserted it in the PC slot, booted up, and I was connected immediately; it works fine. The lowest price I found, by far, was at Buy.com: $58 with free shipping.
So if you've got an older PowerBook with a dead AirPort slot, this might be worth a try...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051109144152863