The town of Nogales is split down the middle of the US/Mexico border (Arizona in the US and Sonora on the Mexican side). Think or Thwim blog has a neat compilation of photos of Nogales in 1898, when you can simply cross the border anywhere, and 2008 (with a 15-foot tall fence and border patrol agents every 50 yards.)
Link - via Incoming Signals
It was the first time I walked from one country into another.