There's an interesting problem.

How would one go about checking the actual number of registered .us names that have real websites associated with them - as opposed to those that direct you to an existing site via a redirect service (domain covering), or a "for sale" site selling the domain name, or a search engine holding site or whatever.

Would you take a random selection of 1000 names from the zone file and manually check them?

Would you rely on someone like Google to tell you?

Would you run the zone file through your own script and trap on redirects or other known domain placeholders?

-t