It's become a common joke that the new Xbox 360 Slim prevents Red Rings of Death simply because Microsoft took the red lights out. However, it's been revealed that there is a real preventative measure built into the Slim, as opposed to a simple aesthetic avoidance of the issue. The new Xbox 360 SKU voluntarily shuts itself down if it's not ventilating properly, in a bid to avoid damage from overheating.

If the 360 detects that it isn't getting enough ventilation, it will give the player a warning message before entering shutdown mode, whereupon its power light will flash. The system can then be switched on again once the light ceases to flash. The idea is to increase the lifespan of the console by powering down before it gets too hot and suffers internal damage.

More: http://www.destructoid.com/xbox-360-slims-shut-down-to-prevent-red-ring-of-death-177121.phtml