I don't get the Time Capsule. Sure, there is less clutter and it's a cool concept, but you pay a premium for the technology. I thought about doing the Airport backup with Time Machine, then decided it wasn't worth the effort. Why plug a hard drive into Airport when you can just as easily plug it into a port on your computer? I would rather send my backups to an internal drive, or to an external via firewire than to send the backups wirelessly and clogging the network. It seems to be an inefficient method of backing up but I might be missing something. The only good thing I can think of is if you wanted to setup a file storage area that all computers could access. This way, if you had friends over, they could access files you have stored on the Airport drive and it would act as a server. Then again, I can do that with file sharing on my computer.
Finally, if you do opt to run backups via Airport, wouldn't it make better sense to have the drives external so you could disconnect and take them with you, add to them, or be able to swap out the drive if needed?