So I tried to dig through this whole article looking for a fix to my problem, but to no avail... can't say I read the whole thing, so maybe my proposed solution has already been brought up.
Just set up a new iMac with Snow Leopard and a new airport extreme... had been having issues with the old airport (and I knew it was the airport because all of my wireless goodies were getting booted, not just the MacBook). With the new iMac set up, went ahead and reinstalled 10.4 from scratch on the MacBook for a clean slate... and immediately ran into the issue of having full signal, but nothing could connect. Changed some settings on the airport to no avail... then decided to tap into my neighbor's unprotected network, and had the same problem. Since every other piece of equipment in the house works fine with the new WiFi, I can only assume, as many have, that the issue is with the MacBook itself. On restart it would find the network, then lose connectivity within 3 minutes.
I found my fix on another website... figured it was worth a shot since nothing else was working.
I turned off my bluetooth, and I haven't lost signal yet. For me it's not an issue, since it's my "sit in front of the TV and browse" machine.
I'm certainly no technical computer master, but it worked for me.