The on-board computer will be a Gumstix Connex with a robostix and netMMC expansions. The robostix can be used for GPIOs, servos and analogue inputs. The robostix provides two TTL level serial ports from the Gumstix for the GPS and phone.
The netMMC expansion has a network port for development and an MMC card for storage. It probably uses up too much power and space for the final thing, but it'll be great for development.
GPS will be used for position and altitude. I have a Garmin GPS-25. It is a simple OEM unit, which will reduce weight and size.
IR thermopiles will be used to detect the horizon for attitude information. They measure IR radiation, and since more radiation is emitted from the ground than the sky, the position of the horizon can be calculated. With 6 thermopiles positioned around the glider (2 for each axis), the attitude of the glider can be calculated remarkably accurately.
See other parts of the wiki for more information on communication, especially radio.
We can use a Nokia 5110 with serial cable to send us a text message of the payload's location and status after it has landed, and maybe put up coordinates on the website using a Siemens M20 GSM terminal.
Not a great concern right now. :)