So if you are like me, after you watch a historical movie, you are wondering what's really true and what's not. (If you really want to know something and a quick search doesn't answer it, someone over at the Lincoln discussion group will know!). So what I was wondering about was the "stories" Lincoln told throughout the movie, including one about Ethan Allen. And that one is TRUE:
My favorite scene in Steven Spielberg’s Lincoln takes place in the War Department’s telegraph office, as Lincoln and Stanton are waiting for news from Wilmington, NC. Lincoln decides to tell the assembled staff a vulgar story about Revolutionary War hero Ethan Allen. Stanton, already on the verge of bursting with the tension, can’t handle another of his boss’s rambling yarns and goes scurrying off. As the room erupts in laughter, the camera cuts to a portrait of Washington hanging overhead. The first president’s stern face gazes down impassively on one of his unlikeliest successors—an awkward, unpolished frontier lawyer—who’s cracking up at his own off-color joke, seemingly oblivious to the fact that he’s sitting at the nerve center of a vast war machine.
As it happens, the anecdote in question is one the historical Lincoln actually told on at least a couple of occasions.
This site goes on to talk about the sources for this story as well as others. Enjoy!