It seems OpenBTS only works for 2G. Hopefully Bellard will open source his LTE base station software in a few years, to accelerate the rise of such networks all over the world:
My understanding is they do support 3G but don't use the GSM standards for 3G networking to do it, they run IP directly over the air interface with a more direct protocol.
That is way cool. The idea that the baseband processor should handle voice audio paths and mobile network signaling is a very "bellhead" idea. With all-IP networks, doing voice payload and signaling in the user space of the main CPU is fine, and could result in more telephony apps based on being able to access voice and signalling more readily. Like, for example, secure calls.
So it is an actual GSM network, and it runs OpenBTS. Awesome.