This is why the headphone jack is small:
Look at how the edges are curved. Look at how close to the top/edge the bezel around the glass is.
Now ask yourself: How much thicker would the iphone have to be in order to make that inset jack wide enough to accomodate most headphones? The answer is: noticably.
The jack on my Sony MDR-XD300's is almost exactly the same thickness as the iphone. In order to have similar casing around the jack, you're probably looking at a minimum of another 1/4" on the thickness of the iphone, and that takes it from sleek to clunky real quick.
So take a look at the iphone, specifically it's thickness and how it's constructed around the headphone jack, and realize it's probably not a conspiracy.
custom pc, sony vaio, mac mini, ibook 2k7, Windows Vista