It could be done up until recently, not sure if still possible but you can try it out.
Add a new GamerTag to your 360 & sign up for XBOX Live Membership. For the email specify a .com address, for the physical address type in a US address (university, library etc.).
If it still works then voila, you'll have access to US Marketplace when you log in using this account.
The different content for US is more to do with distribution/licensing issues.
It worked, I signed up for an account using a random Beverly Hills address.
Not overly impressed with the extra content mind you, I can download HD TV shows and movies elsewhere - I thought they might have some game demos we don't, but that doesn't seem to be the case