I have external speakers and microphone hooked up to my MacBook Pro running Snow Leopard and an iPhone 3Gs connected to it with a USB cord. Is it possible to route calls from the iPhone through the external speakers and mic (with or without bluetooth)? Put another way, is it possible to use my MacBook Pro, external speakers and microphone as a speakerphone for the iPhone?
There are several bluetooth solutions, that should be useful:
Phone Amego ($29) - This should do the trick. Works and has interesting features. I recommend it.
BluePhoneElite 2 - No longer developed and supported. I see almost everything vanished, but I think you should be able to locate a working copy somehow.
Edit: As of OS X 10.10 Yosemite and iOS8 this will be included in the OS itself. It's called Continuity.
ps. I'm not in any way connected to the authors of the software, mentioned in my answer.