Apple - Support - Discussions - Apple iPhone 3.5mm to 2.5 mm headset ...Now I know they work I'll order a few more. I might look for a cheaper price!
I bought two 3.5mm to 2.5mm headset (earset) adapters from 1800mobiles.com part number APLIPHONEHFA2. They work, I can now use my favorite Shure easets with my iPhone 2. They're very slender, so they don't add significant bulk to the earsets. $8 each, white in the picture, black when I got 'em.
If you search on this part number you will find multiple vendors, probably all the same company. 1800mobiles wasn't bad, but they sent me an invalid tracking number and didn't answer customer support emails.
Here's one vendor's description:
Apple iPhone 3.5mm-2.5mm Headset Adapter APLIPHONEHFA2
...for the iPhone 3.5mm recessed stereo audio jack, fits Apple iPhone and Rim 8300 Curve cell phone models by converting the jack to a standard 2.5mm jack for use with any universal headset such as Jabra or Plantroinics or Shure.
* Apple iPhone Adapter fits the recessed Apple iPhone audio jack, since the Apple iPhone headset port is deeper that the outside casing. This Apple iPhone headset adapter is custom designed to makes it easy and convenient for you to connect your standard 2.5 mm mono headsets into the iPhone's recessed headset port.
* Featuring a sleek design which isn't big or bulky, simply plug one end of the adapter into the audio jack of your Apple iPhone, and the other end to your existing 2.5mm headset and you're good to go.
* 2.5 mm to recessed 3.5 mm port Headset Adapter for the Apple iPhone.
* White color cord is 2 inches long.
Update 5/19/2010: After about 8 months of use these adapters fell apart. Then for another six months I couldn't find any working replacements. I tried several that failed. The iPhone 3.5 mm plug does not appear to be a standard connection; I wonder if it's patented and connectors need to be licensed. Recently, I found another adapter that works. I've ordered three of them.
Update 7/12/11: I broke one of my Shure headset/earsets and tried the adapter with another. Same model of earset, but on this one the adapter didn't work as well. Studying it, I'm thinking the problem with these adapters is an antenna/radiofrequency problem, not a mere connection problem. Modern phones are a bit of hellish RF environment, and earsets are a form of antenna. Antennae don't like "couplings". If I'm write, this might explain why these adapters have faded away. Maybe they never really work reliably with a high RF phone.