Matt Yglesias, Brad DeLong and the Volokh Conspiracy (at the moment, Tyler Cowen and Eugene Volokh) ponder the “alternative history” question of what the world would be like without a successful American Revolution. I don’t have much to add, but it’s an interesting concept to ponder. My gut feeling is that separation was inevitable by 1820 or so; the resources of the period (most notably, the lack of real-time communication and fast intercontinental transportation) probably just couldn’t sustain any form of unified government over an area separated by thousands of miles of sea.