Do these headphones work with the iPhone 3G

They do. Now if you have to get an adapter at all, I don't know as the iPhone 3G has the 3.5mm headphone jack. These are way overpriced and only work if you have a small ear canal as they usually fall out of your ears where you have to hold them in. They don't handle music at loud levels that well either. They break more than cheaper headphones that I've come across. I wouldn't waste my money on them if I were you.

If i were you you should do what i did,

Buy a shuffle which comes with those headphones, swap out the head phones with your original ones from your iphone and gift the shuffle with the older iphone headphones, its only a $20 difference between the headphones and a shuffle, win win

http://store.apple.com/us/product/MA662G/B
These are the new headphones that came out with the most recent ipods. They are the ones with the volume buttons on the headphones. Do they work with the iPhone 3G?

Are you sure that is the correct link? There is no mention of volume conrol. They seem to be exacty the same as the ones that come with the 3G iPhone execpet there is no microphone or control button.
<http://store.apple.com/us/product/MA814LL/A?fnode=MTY1NDA1MA&mco=MjM2OTAwMA>
They are the same price too.

You probably mean the new Apple In-Ear Headphones
<http://www.apple.com/ipod/inearheadphones/>
<http://store.apple.com/us/product/MA850?intcmp=AIC-WWW-NAUS-BUYNOW-FOOT-INEARHE ADPHONE-080910>
They are not yet available ("Coming Soon"), so nobody knows if they will work with the iPhone. Probably everything but the volume control will work. They do have a microphone and control button.

Meanwhile you can get the Sennheiser MM50 from Apple
<http://store.apple.com/us/product/TR530VC/A?fnode=MTY1NDA1MA&mco=MjM2OTAwMQ>
(The cord for the second earphone goes behind your head, not undr your chin.)