Apple has now filled what was a gaping hole in their Time Machine backup ecosystem. You can now use Airport connected HFS+ Formatted USB hard drives to backup your Leopard computers. Apple had promised this feature long ago and then pulled the advertising for the "Airport Disk Time Machine Backups" and hasn’t able to deliver during the first two releases of Leopard. This might have been more difficult than originally planned (though our Flux Capacitor has helped many).
Apple has now released Airport and Time Machine updates that now allow you to see Airport connected USB drives and back up to them like Time Capsule drives.
It isn’t known whether Apple waited to sell a few Time Capsules before releasing this fix or the company genuinely didn’t have all of the pieces together. Anyway. Download and backup to your heart’s content.