Holy crap, someone's doing this for real?!
Holy shit, just looked this up. What a great idea (instead of paying $$$ for a Time Capsule).
The only issue is that Raspberry Pi has its Lan connected through the USB internally, so the USB drive and Lan share bandwidth - which means that my backups run at 5-6MB/s, which is not ideal, but works just fine. Just leave it for a few hours when you do your first backup,and after that it runs completely fine ;-)