This should be confirmed via official sources, but discussing with a CFI at the flying club here...
To be PIC in an N registered aircraft, you need to have a US pilots license. My CFI has done so, and said that he was only required to do a knowledge test (no flight test) to make sure that the slight differences in airspace and procedures are understood. The plane doesn't know when it crosses the border or when a new registration is pasted on the side... but ATC does
.
He said it wasn't particularly difficult if prepared.
Of course, different FSDO's may make up their own rules, so it should probably be checked with them.