I think if you build it up as a US based search engine, that would be one way to go. Considering the extension, it might be the way to go. But, my problem is this. If I want to build an SE - and I like the term "SearchNow" - Am I going to buy the .us extension? or am I going to go after the .com?
The truth is, the dot com is undeveloped, I wouldnt dare try to develop a .us without owning atleast the dot com. I think this is a regger (reg fee). Best of luck with whatever you decide to do.