Here's a blurb from the blog-post announcement:

"It senses carbon monoxide and connects to your Nest Thermostat through your Nest Account. So it can automatically shut down the furnace, a possible source of CO poisoning, when the carbon monoxide alarm goes off."

so ... what if your Internet connection is down ?

Further, even if it isn't, why[1] would you ever want your thermostats and/or fire alarms talking to some third-party, for-profit company and existing on public IP space ?

Do I misunderstand how this works ? I hate typical smoke alarms with a passion, but I'm not replacing them with something that can be DoS'd or requires Internet to function properly...

[1] Other than blind, stupid pursuit of new and shiny