Yeah, and apparently it's supported by Apple too, although not documented very well.
I originally used my NAS to backup to using Time Machine and a sparsebundle (the same way it uses the drive on the AE), but my backups were always corrupting. Hopefully doing it this way they should be more stable.
Examinus - why were your backups corrupting, and how would doing it over USB fix this? I've been trying to find a USB drive that would work with my AEBS but have struggled to do so. . . (though I'm told it should work).