Yeah, if there is something that connects a HD through ethernet, it'd have to be quite a bit more expensive than just a regular ol' HD of the same capacity, since it would have to be running some kind of very reliable networking software. Also, it'd probably show up as a network volume/file server, rather than a plain ol' external drive.
It probably exists, because I'm pretty sure that's what the term NAS refers to, but there's gotta be a pretty hefty premium over a FireWire drive.
I think you should just go for the best-rated FW400 drive you can get (or maybe a 400/800 if you'll be using it with more than one Mac)