I have been looking for the same thing and my conclusion so far is that I will have to hack a cable and make something myself. From what I can tell, if you used an existing mic/headset or audio/video cable with 3.5mm plug you could cut it and then wire to a stereo headphone jack and mono mic jack.
I have a XLR to 3.5mm cable for plugging a mic into small inputs but it's 3.5mm plug is basically a mono audio connection so it wouldn't work with an iphone. I will need something that doesn't follow the same 4 contact configuration of the iphone jack. If that makes sense. You could even add a switch in there for pausing music since that happens just by shorting two of the conductors if I understand it correctly.
Good luck. If you find something that you see as quality you should post. there's not many, but there are a few people interested in this.
Also, you should check out an app called 4 tracks lite if you haven't already. It looks very promising.