Same with osmocom.org (their site is down atm. though), but they've among many things built a GSM base band stack, running on real phones.

While certainly not intended for every day use, it's very handy to atleast have something for research and learning where you can access all the protocol layers. Just imagine the TCP/IP world if no one could ever send/receive any packet/frame/bits/protocol below the application layer.(i.e. TCP/UDP/IP/Ethernet was just a black box on a silicon chip).

That's basically the GSM world, at least until the osmocom guys made their own stack, running on cheap and old nokia phones.