I've created an Apiary.io page and Github repo for tracking the documentation of the API:
I'd like some help with this. If you'd like to sniff the traffic of the Model S app, you can use Fiddler and your phone on your local wifi network using these instructions: Fiddler Web Debugger - Configuring Google Nexus 7 running Android 4.1 That will work for any Android 4.1 or higher device, with the exception of some manufacturers that have customized the interface very heavily. It may still be possible to install the Fiddler CA cert, but you'll have to figure that out on your own. Same goes for older versions of Android.
I still need to list out the remaining API endpoints and document the return values. The syntax for Apiary is fairly simple, so if you can do Github Markdown, you can handle this system. Feel free to fork and submit pull requests and I'll merge in stuff whenever I can. The biggest thing missing is the authentication flow, which I don't know if that's OAuth or something else (it doesn't look OAuth, though).
My hope is that we can build some cool apps out of this for our smartphones and for the web. For example, we could build a Windows Phone app (which is being worked on) or an Android widget. Or you could build web service to turn on the climate control at set times or after certain triggers. There are tons of possibilities here and I'm really excited to see what people can think up. REST means the API is easy to use and friendly, so there should be lots of us able to produce something against the API.