Most multiple failure 360s are caused by bad power supplies. No one ever sends there power block in when their 360 dies, and if the powers supply kills one it kills the next and the next etc.
I have a very good contact that works with Microsoft in the XBox division. The explanation I've heard is that there is a heat dissipation problem between the CPU and the Graphics chip. The whole thing needs to be redesigned because even the fix of adding the extra fan isn't a long term solution.