If anyone has already mentioned this, I apologize, but I think its pretty cool...
A couple months ago I bought a set of $30 dollar iPhone headphones with built in mic for my iPod classic. The mic allows for voice recording on the iPod, as well as pause/play and track skipping. I really enjoyed the convenience of these earbuds on my iPod, and thought I'd try them out on the Droid. Here are the results:
For phone calls, they do what you'd expect. Answer/End calls. Although they don't prompt voice commands like they did on my LG Dare, which I thought was also a great feature. Callers heard me pretty well on the other end, although I did have to hold the mic closer to my mouth outdoors, as I've seen most people with the iPhone do.
For the music player (MP3s on the phone) and Pandora:
Clicking the mic pauses and restarts audio playback, although with some minor inconsistency. Sometimes it took 1 click, sometimes two, or up to three. The phone seems to go "dormant" at some point and the extra click or two is necessary to "awaken" it, if you will. However, once initiated, each successive click will instantly pause/play without delay.
Another neat feature discovered: after closing or killing all the apps with taskiller or advanced task killer, clicking the mic will automatically start the last music app and the last song that was playing, (although in the media player's case, from the beginning. But I don't think that has anything to do with the headset).
So if you have an iPhone headset, try it out. Enjoy!