sourceApple appears to have been taking an increased interest recently in addressing circumstances which could result in a customer voiding their warranty due to misuse of or unauthorized access to the internals of their devices. Another recent patent application from Apple described methods by which a device could determine and record whether it had been exposed to a variety of conditions including liquids, extreme temperatures, excessive shock, and unauthorized tampering. Repair technicians could later access these recorded "abuse events" in order to determine what may have caused a given device failure.
So it'll not be long before you're unable to add your own hardware rather than pay the great apple and still have Apple Care..