As you’ve probably heard, Leopard’s Time Machine won’t work with a disk connected to an Airport Extreme base station. However, there’s a very simple work around.
Connect the drive directly to your Mac and tell Time Machine to use that drive. Disconnect the drive and connect it to your Airport Extreme Base Station. Connect to the base station and mount the drive. Time Machine will now recognize the drive and start backing up to it.
However, I find that AirDisk in general is very flaky. Even in normal use, I find that it often drops the connection at random times.
I’d be ridiculously careful about this. If you’ve noticed, Time Machine (specifically backupd) won’t start a backup if you’re not connected to AC power. I’m sure that’s a precautionary measure to protect against the computer’s going to sleep during a backup. It’s reasonable to think that this feature was included, and the AirDisk feature removed, because right now Time Machine isn’t exactly great at recovering from catastrophically interrupted backups. I’m told it can handle a paused/resumed backup, like when you put your laptop to sleep in the middle. But I wouldn’t trust it to handle a backup that just fails right in the middle.