Osmocom is working on Open Source implementations of both basebands and cell towers, along with various other cell network components: see http://osmocom.org/ for the project and http://bb.osmocom.org/ for the baseband in particular.
OsmocomBB is 2G only and it is functional only on calypso based phones, which are extremely old phones, most of which do not work on US GSM frequencies (IIRC there is one that does).
For one there is the Openmoko Neo Freerunner (GTA02) which has a US version. It also has a seperated baseband only connected by some serial and audio lines.
