If someone did patent it in the US, it would only affect the US.
One of the few advantages we non-Americans have, being outside your completely rediculous patent/IP/litigation-mad country. Although to be honest, Australia is starting to slide lately, with more and more people choosing wealth over common sense.
Outside of all that, a patent covers a particular implementation of something. Not only that, but there must be no doubt that there was ever a pre-existing thing that did the same job. For instance: Microsoft can patent a particular feature of the XBox, but they can't patent the idea of video game consoles in general. Someone could patent a particular TYPE of arcade cabinet (assuming they invented it, and it had unique function and wasn't a clone of something else) but they couldn't patent ALL arcade cabinet building everywhere.