Several of my clients buy both the .co.uk and .com extension, and ensure that both URL's point to the same place (check out
The Internet Baby Shop - Baby Furniture, Pushchairs, Car Seats, Baby Walkers, Baby Toys and
The Internet Baby Shop - Baby Furniture, Pushchairs, Car Seats, Baby Walkers, Baby Toys).
Unless your website isn't getting picked up by the US versions of the search engines, I generally don't recommend doing anything tailored to a specific Country. If your website is not Country specific, then if optimised properly for the search engines, traffic from all Countries will eventually come.
Of course, for direct traffic from a certain Country, you could optimise your site to include words for that Country. For example, rather than optimising a website for the keyword / phrase "Freelance Forum", you could optimise for "US Freelance Forum". The visitors who searched for the term are obviously looking for a freelance forum in the US, and so there's a high chance that they are residing in the US also.
Hope that helps.