Googling "atmel microcontroller cubesat" brings up a lot of interesting links, but I am not sure what keywords to use to find anything that has been "flight tested"; it seems, though, that members of their product line have been flown (though perhaps not specifically the ATMega328). Have you contact Atmel about this? You likely won't find much or anything regarding using an "Arduino" for a cubesat, but rather an Atmel ATMega328 (you know, the microcontroller that is the heart of an Arduino). So you need to be sure of your search terms; in the end for such a device, you would be creating a custom standalone system (with the microcontroller, whatever you use, as one part of the overall system), and not trying to integrate a complete design like the Arduino board, simply from a weight/mass saving standpoint.