Apple today released AirPort Extreme Update 2008-003.
About AirPort Extreme Update 2008-003
This update is recommended for all Intel-based Macintosh computers running Mac OS 10.5.5 and resolves some issues with AirPort connections when roaming in large Wi-Fi networks.
The update weighs in at 2.2 MB and requires OS X 10.5.5.
Update: Apple has pulled the update for unknown reasons.