The OpenBTS project's aim is to build a open Base Transceiver Station (BTS) that is a mobile/cell phone base station. The project uses a software defined radio specifically the well known USRP. The project seems to be going well since they already had a field trial at Burning Man this year.
This project should be really interesting. Also using this will be rather impossible because of license issues with the wireless spectrum that is used for mobile phone.