Soon before the whole machine stopped working. The first card had already stopped working for a couple of minutes:
$ DISPLAY=:0 aticonfig --odgc --odgt --adapter=all
Adapter 0 - ATI Radeon HD 5800 Series
Sensor 0: Temperature - 127.50 C
Thats a little warm. Did you smell burning plastic?
Sensor 0 or 3 is usually the VRM (voltage regulator module) on the GPU. Temperatures of 90 to 130c are quite usual on air cooling, and the VRM's are specified to take this kind of heat
(esp. the relatively expensive Volterra modules on 6990's which can take temps of even up to 150c without throttling)
On normal cards (non 6990) 135 to 150c will degrade the VRMs very fast or kill the card in 24/7 usage.