I know Monster <shudder> makes one. I saw it at the Apple store a few weeks ago, but I couldn't bring myself to purchase it. I think Shure made one a few years back.
If you want a whole-unit solution, the Etymotic HR-5s (I think that's the number) come with a mic built in.
The HF-2 is the model you're talking about. They have the built in microphone.
Rick
"A fear of weapons is a sign of retarded sexual and emotional maturity." Sigmund Freud