It looks like the first American has caught them all! A guy called ftb_hodor reported his success via Reddit, claiming and proving that he had completed the current North American Pokédex. He caught and/or evolved 142 Pokémon, while the nine Pokémon that remain elusive as they have not been spotted anywhere in North America as of you.
Still missing from his Pokédex are Ditto, Articuno, Zapdos, Moltres, Mewtwo and Mew. These elusive six have not been occurring anywhere worldwide, yet. And then there are three others: Farfetch'd seems to be an Asian exclusive, while Kangaskhan is found on Oceanic ground only and Mr. Mime being the Western Europe Pokémon.
In order to finish his collection, ftb_hodor caught 4,269 Pokémon, mostly in New York using the four hot Pokéstops at Central Park. He hatched 303 eggs and walked 153 kilometers during the past two weeks, reaching trainer level 31. Clearly he'll have to travel the world next...