I'd assume it wouldn't be a timeout, but a higher temperature threshold. If the machine is on the verge of melting already then you wouldn't want to turn off the fans at all.
I bet theres a way to test their cutoff mechanism. Maybe someone could run some serious bench-marking apps (maybe more CPU bound ones) and see if Siri still kills the fan at high temperatures.
Could someone potentially damage their very expensive machine to satisfy my curiosity?
All this could really open them up to is a hard-shutdown, and I imagine they would have implemented this as a higher threshold, not a complete disable -- at least, it seems like a really easy problem to foresee and circumvent.