OpenMoko
GSM Integration

But you can't hack the GSM stack
so you get the maximum level of freedom that you can get with any other peripheral device:
open source low-level (mux, power mgmt) drivers
open source high-level drivers (gsm daemon)
openly documented serial protocol (TS 07.05, 07.07, 07.10)
asking for more freedom on the GSM side is hypocritical when accepting the very same level with other peripheral devices.