For bluetooth:
I use the Motorola H700 ($70) when I drive and it works great. Just discovered though that plugged into the aux jack, the call will go over my speakers. I just talk like its on speaker phone...only its on super speakerphone.
w/ music option:
I use a panasonic adapter ($9@target) with a microphone. Like an extension cord for head phones, with an added microphone. Got it for my first iphone and love it. Plug it into my Sony earbuds (block out other noise and are very comfortable and small) You can listen to music then a call will come into your earphones and you can hit the mic key on the Panasonic, chat, then go back to music. It is awesome.