The only thing is, like keith is saying, it looks like there could be an opportunity as these are showing on enom - but they're a bit pricier, and i think require a 2 yr reg.
as far as it being a sub, yeah you can create whatever.yourdomain.com, but .us.com may actually get out there and get used since it's on the list of results on enom..
There's no doubt I probably wouldn't reg one for personal use, but if they can be marketed for sale I don't see why not.