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...

