OpenMoko
GSM Integration

But you can't hack the GSM stack
yes, that is true.
pretty much like you can't hack the firmware of your SCSI or RAID controller, WiFi card, Bluetooth chipset, etc.
even the firmware of a good old analogue phone line (voice) modem was not hackable
having proprietary firmware on a dedicated peripheral CPU is even acceptable to the FSF!
And no doubt, anyone inside OpenMoko would love to ever have a open source GSM stack.  Patches welcome :)