I never flip people off to their face, it's abhorrently rude. I do, however, flip off every inanimate object and every jerk on the road I see, mostly because if I overreact that way, there's less a chance of me kicking a hole in the wall (which I have done - halfway through an animation when the program crashed and lost all my work...).
As a side note, In Bruges is a fantastic, AMAZING film. Makes me laugh every time. And also want to jump Colin Ferrell, that incredibly hot irish man. Plus, you know, hitmen rock as a stereotype.