An U.S. licensed pilot may fly a foreign-registered aircraft in the U.S. as if it were U.S. registered.

The problem with license and registration only comes about when the aircraft is in it's home country -- then the pilot must be licensed in that country. In the pilot's country of license, the registration of the aircraft doesn't matter.

At least, these are the rules for Canada and the U.S.. (Been looking into it, since my club in Canada is hosting the Women Soaring Pilots Association seminar in 2014... we're expecting a bunch of U.S. pilots to come and have been working with Transport Canada to get everything in place for the visitors).

-John