I said I wouldn't post here but....
Just wanted to say that this is false and I have mentioned this before. We perceive sound distance and direction (out of the head sound cues) only through cues present in the sound pressures at the two eardrums. Also, influence of the shadowing of the torso, head, and pinnae on the frequency spectrum perceived at the eardrums also influences these cues. Research has shown that direction in the horizontal plane is almost entirely determined through pressure differences between the two ears and that direction in the vertical plane is determined through comparing the frequency spectrum of the sound source to previous experience with such sources at known directions. Both of these key factors for localization are contained within a recording unless the recording is seriously flawed. Sealed headphones work just as well as open headphones to perceive these cues and under certain conditions one will be better than the other. If the sound doesn't make it out of the cup and you perceive no "out of the head" cues, it's not the headphones...it's either the recording or you.
Aww, well, we knew you'd be back ;)
Yah, that's pretty much the case. Closed cans will really only effect the back pressure on the driver and the amount of reflection. The key is to get your headphones to reproduce the recording exactly how it was mastered (hence all the damping matting and sealing inside the cups!).