The old tartley.com used to have one of these, so here's a quickie replacement. It tends to overstate the case somewhat, a single foray across a border floods entire countries. I'd imagine good old Google's freebie geospatial tools could be put to use to create a better one, but for the time being, here's an amusing approximation. I'd be amused to see Skankmeister's version. Layovers don't count - bone fide visits only.
28 countries (12.4%)
24 states (48%)