Rob
I used to own a handful of "whatever.us.com" domains, including mobi.us.com, and some others
basically though, I agree with Rob from the aspect of it being a 'subdomain' and that your usage is dependent on the owner of us.com domain name
still, some of them got type-in traffic and earned ppc revenue, as I picked them straight from the Overture tool
I wouldn't advise going that route now though
imo...