The iPhone 1stGen headphone jack is surrounded by a metal sleeve that prevents most normal headphone plugs from fitting. I modified a ~ $7 Radio Shack adapter to let me connect the headphones I already owned to an iPhone.
This picture shows the white Apple iPhone headphone plug. Note the narrow white plastic sleeve where it joins the metal shaft. The black jack is my modified Radio Shack 42-2570 connector.
And here is the modified adapter in use:
With a utility knife I carved away a small section of the black outer plastic, then the waxy white inner plastic. Done.
by David Phillip Oster