The design of the 360 case is inherently poor for heat issues as well. Setting the console up vertically, while you have a HDD attached, blocks a significant amount of airflow. I recall reading a survey (I wish I could find the link) which implied that most of the "living" Xenons/Zephyrs had been horizontal through their lives.
Very likely, a huge number of users are mostly unaware of the amount of room a modern CPU/GPU need to breathe in, and the X360 was mostly just a victim of unaware users placing them poorly, and not understanding the serious heat issues modern computers have. The PS3 has a much better case design, in this regard... probably as a result of Sony's lesson from the early PS2. I would not be surprised if the next XBox was rock solid, from this standpoint.
Although... MS has stated that they want to be first again.
