its like domain parking sort of thing

Google wont know about your hidden domain untill you tell him , its redirection is for just users

They're not treated equally, though, by various browsers. If I just type in the part of the domain name before the dot into my brower and hit Ctrl-Enter, the browser defaults to .com.

Ditto, most users probably think .com automatically.

That said, many sites do extremely well with .net, and there's no reason that .us should be any different. But so far, .us hasn't really caught on. I assume that eventually it will become more common.

Generally, I advise clients to get, if they can, the .com, .net, and .us versions of their domains, as well as at least the .com and .net hyphenated versions, to keep competitors from using them. We then 301 redirect the extras to the main domain.