I remember some U.S. Americans getting their collective teet in a wringer a couple of years ago because of this. They were leaving out of Canada and heading to Cuba. Back then you didn't need to show a passport to get back into the US if you came across the US/Canadian border over land. Now you do and the agents are going to wonder what that big Cuba stamp is doing on your passport. Back then I don't think it was a big deal because these people didn't have to show their passports to any US officials. That has changed and you would probably get in big trouble, hypothetically.
Moe hit the nail on the head. Before you had to show your passport coming back to the US from the north and south, simply going to one of those countries and flying to Cuba was a hypothetical option. Now, however, not a hypothetical option because you have to show your passport, you also have to write down where you've been and the customs official asks. If they were to look at your last couple stamps (which most do), your hypothetical trip to Cuba would come with a nice hypothetical fine, more so than those hypothetical cigars and rum cost while you were down there! You can find pictures on the web! I suggest doing a hypothetical vacation there.
Besides, IMHO, what happens if you got sick or something happened down there?
