I think the dark gray one was some sort of "aero buck" or styling test in foam or clay. They did use it to leak teaser photos before we got to see the white and silver ones which had evolved a bit.
More here:
http://www.teslamotorsclub.com/tesla-model-s-whitestar/2487-model-s-prototype-front-ends.html
Some terminology I recall hearing:
Buck: A model of the car's shape used for wind tunnel testing or styling contemplation. (Probably the Dark Gray model S).
Mule: An existing production car gutted, and then reskinned to look like the expected new car. (There was a yellow Roadster Mule 1 that looked just like an Elise with a Tesla drivetrain, and there is a Dark Gray Mule 2 that has an Elise chassis but Tesla drivetrain and custom bodywork that looks a lot like the final Roadster). In some ways the Silver (now Red) model S is a Mule. It might even be "Mule 2" with some other test car out there looking like a stock CLS (or somesuch) with a prototype Model S drivetrain. Remember the white Dodge Magnum wagon chassis we saw left out in plain sight? That might have been a retired mule. Or a decoy mule.
Concept: A completed, drivable, hand-made vehicle done as a styling exercise to gauge public reaction. Some have called the Silver (now Red) Model S a concept car. With that sort of moniker, it would mean to me that the production Model S might differ substantially from what we see now.
Glider: A completed chassis with no drivetrain. (For examples the Roadsters shipped from England to California to have the drivetrain installed).
Roller/Pusher: A sample show car with complete exterior but dummy interior (e.g.: the White Model S)
EP (Engineering Prototype): The first attempt to make a batch of production ready cars. (Used in crash testing, and engineering debugging).
VP (Validation Prototype): What is thought of as a completed vehicle to be handed off to marketing to give test drives and solicit final feedback for fine tuning.
So, I think the real big milestone for Model S would be when some EPs are produced. I don't know if they would try to make any before or after the factory is actually built out and tooled up.
I am guessing that a lot of the above terminology came from Tesla's early involvement with Lotus engineering. (Do they still use those same terms now for Model S development? Is Lotus involved in any way with Model S?)