Yes, now that NEST has opened their API (i.e. their control protocol) for outside development, technically Crestron or any other third party system can work the thermostat right now, today, if the command string protocol was published. The way a lot of companies are going, Nest will have private conversations with Crestron, for example, and Crestron will write a bulletproof "module" for controlling the Nest thermostat or Nest will take it upon themselves to create a drop-in programming module (i.e. app) to control the device. But given the demand I wouldn't be surprised if we saw Nest integration availability with all major automation systems within just a few months. Control4 just happened to be the first because it is the first real
automation system that is also affordable. I can only imagine the sales that Nest had not captured to this point by being a stand-alone device, closed to other control methods.
The one thing Nest seems to be pretty insistent about is that the thermostat remains in control and will override control system commands to some extent as part of its "learning" of its users and where they want settings. Here's a quote from this article
Integration is via Nest’s cloud service, which pushes the thermostat states to (in this case) Control4’s cloud and ultimately to the home’s control system.
The learning features of Nest are not incorporated into the driver, and in the event of conflict between Control4 and Nest … the thermostat wins.
You can’t schedule the thermostats through Control4: “If I start externally mucking with their [Nest’s] schedule, there’s no telling what I would do to their own heuristic schedule,” Anderson says of their algorithms Nest builds around user behavior.
If a consumer must have the Nest thermostat, they will be able to integrate with Control4, but the richest experience is reserved for Control4’s own thermostats.
For instance, in Control4 if you change the temperature, the system asks if you want to revert to the usual setting in two hours or some other time period. The feature is built into the firmware of the thermostat. No can do with Nest.
“Technically you could do it in code, but it’s relatively cumbersome,” Anderson says.
But hey, at least it's a solid start to outside integration and some of the macro commands I can imagine you needing when running your theater as it pertains to HVAC.