A seriously cool idea this; making a smartphone out of a Raspberry Pi.
A guy called Dave Hunt put together the PiPhone, using a 2.8 inch adafruit TFT touchscreen and a Sim900 GSM/GPRS module. The result is pretty sweet as you can see from the video.
Dave did a PiPhone writeup on his site if you fancy having a crack at making your own. I actually have a Pi kicking around; maybe I will.