On: At Defcon, hackers get their own private cell network: Ninja Tel

[tonfa]

Also HAR and I believe Burning Man had GSM networks too.

[bsho → tonfa]

Burning Man's GSM network used the GNU Radio project:

http://gnuradio.org/redmine/projects/gnuradio/wiki

The Suggested Reading section gives a really good starting point to DSP, electronics (including Radio and RF design), and Programming (C++ and Python). It's been a great help to hobbyists like me.