And it is live:
http://www.nest.com/blog/2013/10/08/meet-the-nest-protect-smoke-and-co-alarm/
Still trying to figure out how much use the thermostat will make of having additional sensors in the house - so far the only integration point I've picked up on is that the smoke alarm will use the thermostat to connect to the outside world....I hope there is a lot more.