I planned a similar project.
At minimum I want GSM / GPRS functionality for phone calls & text messages, LCD touchscreen and of course battery operation.
I already ordered this: http://www.mikroe.com/eng/prod.....ttm1-board
which looks quite promising but on the other hand needs @ minimum 9V supply.
I received it yesterday and found out that with 5V supply voltage it still can drive the TM1 module. (The LM2576 has VinMin=4V).
The question now is the efficiency of the step-down converter and whether it still can provide enough output-power.
Writing software to control it will not be that hard considering the AT command-set and the RS232/SPI support.
MIC-In / Speaker Out is already on-board too, so the Pi-Software would just be used to control the GSM chip.
In addition, something like this: http://www.sparkfun.com/products/10300
could be used to charge the battery + providing 5V supply to the Pi & SmartTM1 boards.
What remains uninvestigated is the need of some kind of "power-down-mode" for the ARM11 core and a wake-up functionality – "wake-on-call"/"wake-on-text", else the battery would be drained pretty fast.
Ah and btw.: Can anybody confirm or disprove the rumours that iPhone Touchscreens are compatible with the Pi's DSI connector?
Looking forward to your oppinions!