BlueAnt Z9 headset - awesome with iPhone!
http://www.myitablet.com/my-itablet-review-blueant-z9-bluetooth-headset-151384.p hp
I decided to pick one up. MyiTablet sells them for $99, a full $30 less than the iPhone headset. It's not quite as sleek or simple as the iPhone's headset, but it's still quite small. I found I actually really liked the size because the buttons are really easily accessible, moreso than on any other headset I've used in the past. The volume adjustment works fine with the iPhone and it paired very easily. One feature I really like is that you can pair up to 3 devices to the Z9 headset, then press the multi-function button to switch over to them. That way if you wanted to swap the headset from using it with your iPhone to using it with, say, your Mac laptop over Bluetooth to chat on Skype or iChat, you don't have to do the whole pairing dance every time. It has a lot of other nice little features, just read the review linked above for more info on those. Oh, it also has great range - I could walk all over my room without any static at all.
Overall I really like it. If you need a new Bluetooth headset or are frustrated with the one you have, you'll probably really enjoy the Z9 headset. It's loads better than the iPhone headset in terms of quality, you can adjust the volume easily, you can swap between Bluetooth sources quickly, and it has great range.
Mac OS X (10.4.4)