There is a semi-official way to control the Nest thermostat via Echo.
You have to use IFTTT. It's not a great way to do it, since you can only say one particular phrase to set the Nest's temperature, i.e. "Alexa, to-do, set temperature." IFTTT will read the to-do list and get the "set temperature" command. Then it'll set the Nest to a pre-defined temperature. You won't able to say things like "Alexa, set the temperature to 78." I didn't find this particularly useful, so I don't use it.