T-Mobile has started shipping a 3.5mm headphone jack with the T-Mobile G1.
"The G1’s are now reportedly shipping with an in line adapter that acts as an extension to he headphone cord itself. This adapter adds length to the cord with a more natural look as if it is not there at all. The adapter even includes a microphone and answer button so users can use the headphones as a headset as well." Via Android Community .
If you already own a G1 you should call T-Mobile customer support and request they ship you the new adapter.