Using the 10.4.8 kext with Leopard gives you a working airport extreme, but only for WEP or open networks. WPA is proving a bit belligerent, you can connect to wpa2/wpa networks using the "chipxsd method" described at the end of this post. Editing the 10.5 kext can lock up your system when trying to use your airport. Feedback would be greatly appreciated if you have found a way, or even if you tried and failed.
sudo chmod +s /usr/sbin/networksetup
sudo nano /etc/rc.local
networksetup -setairportnetwork "yournetwork" "yourpassword"