Well this is not up-to-date but as it's a high toped google link, I'll just put some corrections. As we know now, it's NOT possible to time machine to any airport disk. Only to Time Capsule.

And secondly, you can't attach your drive locally then put it on another computer (not airdisk I just said it) and do a time machine backup over network, because it use another way to do it. Locally it makes a folder and work in it, over network it creates a sparse image file.

Worst, you can't do a time machine backup on your time capsule with ethernet plug then via airport, because it use the mac address... Well for that I have to say it's not nice... I tried network backup, it's not wrong to say 80GB needs btw 12-24 hours. REALLY LONG. :)