After having had some performance issues with 2.4ghz Xbee modules I just finished up a quick project for long range two-way communication. The system is based on HACCOM HAC-LM96 500mW 433mhz 9600 baud radio modems. Range is 2km (1.2 miles) ground level, so the system should be good for really long range communication when the plane is up in the air.
HAC-LM96 radio modem - $52
Turnigy UBEC - 3A switched DC-DC converter - $8
Teensy 2.0 - Arduino compatible board - $19
To make it plug-and-play with my Futaba 9C radio I took the casing from an old defect RF transmitter module and put the UBEC and Teensy board inside. The UBEC is connected to the 9C V+ pin (9.6-12V) and supply 5V for the Teensy board and the HAC radio modem. Sadly the HAC modem is slightly to large for the transmitter module casing, so i had to use an extra box for the HAC modem.
The Teensy MCU converts 8 channel PPM from the 9C radio into serial data for the HAC modem. The Teensy board is my favorite Arduino compatible board, has more i/o in a smaller formfactor and a proper USB interface.
In the airplane the serial data is received by the HAC modem and either transmitted directly to the autopilot or converted back into servo PWM pulses using another Teensy board. This way I can use the HAC modems as a pure R/C control system if I need to, freeing up the 2.4ghz band for live video link etc.
To use the system as a two-way telemetry system you connect the Teensy board in the RF transmitter casing to a computer using a USB cable and either talk directly using the USB interface or use a virtual serial driver for normal two-way serial communication.Edit
: Here is the source code for the Teensy PPM decoder and rf433 transmission system.Teensy_PPM_Decoder.pde rf433.h