Using a 3G GSM shield and the APM2.5+, based on the Arduino Mega 2560, create a serial telemetry connection over UDP accessible over the internet at the assigned IP address. The Mission Planner software must then be able to connect to this IP address allowing bidirectional communications with the APM2.5+ running the latest version of ArduPilot (2.68 as of this posting).
iComSat 1.1 based on the SIM900: http://imall.iteadstudio.com/im120417009.html
APM2.5+ Arduino Mega 2560: http://store.diydrones.com/APM_2_5_Assembled_p/br-apmpwrkt.htm
Mission Planner: http://code.google.com/p/ardupilot-mega/downloads/list
Based on this project: http://diydrones.com/group/telemetry-over-cellular-ip
Partial instructions: http://diydrones.com/group/telemetry-over-cellular-ip/page/testing-the-dronecell-or-other-gsm-modem
Programmer will provide a solution that details exactly how to wire the modem to the APM as well as a modified version of the ArduPilot code with instructions for updating future releases. All findings will be published for public use under the creative commons license.
If you are capable & interested in this project please contact me and we'll work out the payment & hardware arrangements.