Stability control works by modulating the brakes on different wheels. So I expect they can balance the open differential using that system.
Doug, I agree (and have owned cars which used that strategy as part of the traction control system).
I live north of Toronto, where we get more snow and colder weather than the city, so I have been following this thread with great interest. We also have a significant hill which is often unplowed when I have to drive to work. I have had the Model S out a few times in snow and have not, to date, been able to replicate the problems reported by some of the contributors to this thread.
Unless a car is hanging up on the snow (which should be less of an issue for the Model S, due to its adjustable air struts) traction in snow is a relatively simple engineering problem to model and address. The principal variables are the friction and the snow moving characteristics of the tires, the width of the tires, the balance of the vehicle weight between the driven and non-driven wheels, and the characteristics of the power delivery system (the mapping of the traction control, ability to modulate torque at the limits of adhesion, limited slip functionality, etc.). Over the years I have owned and driven a wide variety of vehicles in the snow on our hill. These have included high powered rear wheel drive vehicles (e.g., SS Camaros) which were intrinsically much worse that the Model S in snow due to low (and non-adjustable) ground clearance, and minimal weight on the rear wheels, but could work very well in the snow with the aid of a couple of hundred pounds of weight behind the axle (thereby giving them a weight balance roughly similar to that of the Model S) and some skill with the clutch and throttle. I have found that different strategies are required depending on the snow condition and underlying road surface. In most cases you want to be able to continually push the tires right up to and slightly beyond the limits of adhesion (but without generating excessive wheelspin) - limited torque, with good modulation through the accelerator, combined with modulated braking to limit the slip, as Doug sugested, would work well in these conditions. In other cases, spinning the wheels to get down through a fresh snow cover to sand covered pavement is the better strategy - traction control off, except for some use of modulated braking to ensure that both wheels are driving, works better in these cases. Both modes, and possibly others, could relatively easily be implemented through software on the Model S. The level of regen should also be dialed down in the "snow mode" configurations.
As discussed above, tires are the other important variable. While I haven't yet had any of the problems that others have reported, I was surprised to find that the Model S' winter tires were the same 245 width as the summer tires. For rear wheel drive cars, I have typically selected winter tires that are at least 20mm narrower than the performance summer tires used by that car. The wider the tires, the more work that will be required to push the non-driving front tires over and through the snow. I wonder if the use of slightly narrower tires might help to address some of the issues reported in thread (including high energy consumption in snow)? In this regard I note that Tire Rack offers a couple of 235/50R19 winter tires for the Model S, in addition to the 245/45R19 size. While a narrower tire may well involve some compromises in handling and/or wear, better performance in snow may be worth it to those in snowy areas.
Bottom line, it should be possible, with relatively modest changes to the programming (and perhaps the selection of some different tires) to make the Tesla a very capable car in the snow, and a lot better than most other rear wheel drive cars (due to it rear weight bias, adjustable height and fully programable power delivery controls).
I haven't encounter any fogging up or icing up problems, but usually drive the car solo and have pre-heated the interior (while on charge) which together with a high fan speed bringing in outside air likely helps to avoid such problems. The energy consumption on the road is also significantly higher in winter (which I had expected from my experience with Prius'). When I tried to minimize energy consumption, driving about 80 km/hr in range mode, I managed to get the consumption down to about 200 W/hr per km, but after 40 or 50 km I received a cold battery pack warning, even though the outside air temperature was only a few degrees below freezing and the battery still had close to 300 km of range left? Does anyone know the implications of that warming? I arrived home shortly after receiving the warning, put the car on charge and all seems to be fine, but the warning seemed to be a cause for some concern (especially in view of the relatively warm temperature and short duration of the trip).
