I had this problem with my Samsung a930. I'm no electrical engineer, but here's my theory on the problem:
The problem is the third ring on the connector. If you look at the plug on the headphones they sell specifically for cell phones, you'll see a third ring on the stem of the connector. One of those rings is for the microphone on the headset. I think this also might have something to do with the hangup/pickup button on some of them. Most stereo headphones have only 2 rings. So when you plug in the stereo adapter, one of the stereo channels is going into the wrong "ring". Stereo adapters don't have to deal with this except with cell phones. I don't know the actual configuration of the 3 ring setup, but if someone could post a diagram, one should be able to create an adapter that sucks both channels out of the phone and into a standard headphone adapter.
I ran into this problem when I wanted to have my Samsung a930 play through my car stereo's tape adapter. If I get my hands on a cheap headset, I may play around with it to figure out the configuration. Apparently, however, they aren't standard.
Google Groups Search for this problem:
2.5 mm 3.5 mm adapter cell - Google Groups
Good luck!
-Capey