To those screaming that it works right now with AEBS and an external disk, my reply is, "no."
I have tried to use it a couple times, and it proved to be unstable. Twice, my backup image got corrupted and unusable. That is NOT a reliable backup solution. It is obvious that this is why Apple disabled the feature, to avoid widespread data loss.
The main reason for this is because Time Machine creates an image on the networked drive, and opens it when connected, then writes to it. Given the AirPort and AirDisk's less than stellar reliability, when (not if) the AirDisk disconnects, even for a second, BAM, the image is corrupted, and your backup is no more.
I do believe that this is a limitation of the AEBS. Perhaps a software update can fix it, perhaps not.
The Time Capsule does not use USB to transfer data from the external disk to the AEBS, then the AEBS transmits the data wirelessly. The HD is a actual networked drive. And thus will put a lesser burden on the AEBS itself, and overall, will be a far more stable experience to the user. The NAS is a proven solution, the USB - turned - networked drive is not.
Notice how Time Machine currently works with networked (NAS) drives without any hacks. But requires a hack to work with a USB-attached drive (AirDisk). The reason is obvious.
And hence, Time Capsule.