I just upgraded from 10.4 to 10.5 and my Airport meter is now reading 2 bars instead of 5. Same router, same position, different OSX only. What the heck?!!
Well I bet you did the simple upgrade procedure and got bit in the Airport upgrade bug.
So follow these steps in order or you will be sorry!
1. Turn off the Airport signal.
2. Mosey to /System/Library/Extensions/ and drag out the files AppleAirport2.kext & AppleAirportFW.kext to the desktop.
3. Restart immediately!!!!!!!!! It will take a while rebooting because it is rebuilding the wireless, so go get a coffee or something.
If this doesn't work go to System Preferences (in your Dock)->Network and make a new custom "Location" (calling it what ever you want). Then reconnect to your wireless and throw out the files you dragged to the desktop. Also in the Network System Preferences pane click or the "Advanced" button. A drop down will come down and there click on the TCP/IP tab and there turn off IPv6.
These were the simple fixes for that stupid Upgrade bug to Leopard problem. Good Luck.