After the 10.6.8 update the Airport file permissions were possibly not returned to their original state.
Open Disk Utility in the Utilities folder, select your boot drive and Repair Permissions, enter the admin password if necessary.
Log into a admin account and run Software Update so Airport gets updated. Also toggle your System Prefrences > Secuirty > General > "Auto-update safe downloads list" on and then off.
After that download and run ALL the cleaning and maintenance aspects of the free OnyX and reboot.
http://www.titanium.free.fr/
And then use OnyX > Verify > Preferences (check the box for show only corrupted ones) and write down all the corrupted .plist files.
Use the free Easy Find to seek and Trash those .plists (or move them to the desktop) and reboot. They will get automatically rebuilt (minus your preferences of course which you need to reset.)
http://www.devon-technologies.com/download/index.html