I've been doing P90X over the summer since I won't have access to the campus gym till class starts up again in the Fall. I use the Iron-Gym pull up bar that Paulflexes alluded to and no, it will not break the door frame. It's constructed so that some of the weight gets distributed more horizontally than veridically.
Some notes though: use your common sense, if the door is too wide and the bar's support pads just barely touch the door's frame don't use that door, or else you could be in the middle of a pullup and wham, the pullup bar will go right through the door frame on one of the sides and you might hurt yourself. My mother did this, and I told her to stop using that door frame, but she keeps doing it anyway.
Another note that I noticed from using the bar: overtime the support pad that makes contact with your door frame might wear down, and they also will leave black marks on your door frame. I've seen people use wash cloths and small towels as buffers between the door frame and the bar's support structure. Not sure if its safe or not. Someone back me up and reply as to what you do there.