Yesterday I was co-presenting with Dieter Spaar on Running your own GSM network at the 25C3. The talk went quite well, and received an overwhelming response.
Together with the talk, we also announced, described and released the current development version of OpenBSC, a software implementation of the minimal subset of the GSM BSC/MSC/HLR in order to get a GSM BTS up and working.
The code is available in svn, there's a wiki describing it's current status and features (or lack thereof).