I have an airport extreme with a drive connected, and use it for backing up snow leopard macs. I have an occasional problem with one of them that says "the backup disk image ... is already in use" and it fails to backup.
What worked for me in the end was to "disconnect all users" from the Airport Utility - and then unplug/plugin the drive again (remount it) - it was then able to backup successfully.
There isn't an .InProgress file still on the drive is there? That can cause problems. Just delete it to get the backup working again.
The disconnect, power off/on works, but only temporarily. I think for me it hoses up if I close my macbook pro lid while its trying to backup.