I find it hard to believe that they could have an actual vehicle launch event with press, guests, etc. on 12/17 without more news leaking out. Maybe this is more of a small "preview" event with a live webcast and website update with actual launch events next year.
Since we're speculating. I'll go with a Gen III "platform" introduction blog/webcast/teaser video on 12/17 and that the "SLI\" letters have something to do with the vehicle designations. So maybe a "Model L", "model I", "Model "V", or model "\"??
It has been mentioned in the press that they may be build three vehicles on the Gen III platform (
Tesla Plans Two More Launches: A Smaller Crossover SUV, Sports Car | Autopia | Wired.com). Maybe the Model "\" is the half size version of the Model "X".