This is how it's done: First plug your airport disk directly to your mac and set up time machine to use it. If you want to let it back up now this is fine or you can stop it and let it back up when its plugged back into the AEBS. Time machine creates a file in the root directory of the disk called ".(Some hexdecimal number)" it appears this file has to be in the root of the network shared directory. In my case I use user accounts to manage my airport disk so the directory that it actually shares out is called /Shared on my disk. So I simply move the .23468f43400 file or whatever it may be called to the /Shared directory. If you let it back up while plugged in locally you will also have to move the .sparsebundle file to the /Shared directory. Eject the disk and plug it back into the AEBS and mount it via AFP and time machine should pick it right up. This method should work for any afp share not just airport disks.