My roommate has a dog that is about 2 years old and is housebroken. He generally stays downstairs in my roommate's room, but when he comes up stairs with everyone else, he poops in my room or in front of my room door if its closed. It has happened 6 or more times since he's been living here (since August 2011). What causes him to do this?

It might be that he's set these areas as his "places" to go upstairs.

It appears the dog is not really house trained. Frequent walks and letting outside can make a dog appear to be trained but its actually the owner who is trained. Your roommate needs to crate the dog, close his door or provide direct 100% supervision of this dog. I would also suggest getting a good bio cleaner make for dog accident to rid the area of the scent which he now thinks is his place to go.