Get Time Machine to work with Airport Disk! (And possibly other network shares) – Mac Forums

http://forums.macrumors.com/showthread.php?p=4402934 defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1
 Fining this little gem online this morning let me resuse a 160GB USB hard drive attached to my Airport Extreme for Time Machine backups instead of shelling out $499 for one of Apple’s new Time Capsule NAS boxes.