[floatr]
This was one of the first things we were told when we started to travel abroad. Don't call yourself "American", it offends people because it makes you sound like your country owns all of North America. Canadians get extra furious.
While there isn't a clear consensus, many variants have been used such as "Citizen of the US", USen, Washintonian, Colonican, Columbard, Fredonian, Statesider, Uessian, United Statesman, Yankee, and Merkin.
In Latin American, "Gringo" (white non-Spanish speaking) has come to generally refer to a citizen of the U.S.
Of course all this over looks the obvious. How about we just go with "The United".... it has a nice ring to it and is the complete antithesis of what we are.
[/floatr]