This map, caught by Flickr user littlebudapest, apparently adorns a wall at the Niketown store in San Francisco's Union Square (or at least did during the All-Star Game last month). It divides the United States (and part of Canada) into thirty "countries" (and a large swath of unincorporated mountains and desert) defined by the fan base for each of the thirty Major League Baseball teams. The boundaries seem fairly accurate, at least from my experience (I haven't spoken with that many people in the South, for instance, but I gather from country music that much of it is Braves Country, as reflected here).
As the Flickr photo commenters point out, one glaring omission are the diasporic fans who inhabit the West: some territories, according to anecdotal evidence, follow a specific team from the East Coast or Midwest, even though not everyone in the territory ventured from that team's country. If the commenters are to be believed, much of Idaho prefers the Mets, for whatever reason, and I recall that in the days before expansion delivered the Diamondbacks Arizona was Cubs territory (and remains so for contrarians).
(Cross-posted at The Good Reverend)