Adding a microphone to the iTouch does not turn it in to an iPhone in any shape or form. It's still lacking the actual phone-functionality for starters. You need a bit more than just a microphone to have a phone...
I was actually thinking about this few days ago. Adding a microphone makes sense if you are adding a video-camera. Otherwise the video you would shoot would be silent, unless you had separate microphone attached to the device.
You don't need much more. All you essentially need is:
-Verizon MiFi
-multitasking for having the ability to recieve incoming calls. I believe that's coming next year anyway.
It's not as seamless a solution as the iPhone but you would be saving money on monthly cellular payments and would have free tethering to boot.