Hi, Just thought I'd submit my tuppence worth to this discussion (even though it ended a week ago and there isn't any snow now!).
As far as I can see, nobody seems to have mentioned the fact that the IS200 is rear wheel drive. This has a significant impact on traction as most of the weight in a car i.e. the engine, is in the front of the car over the front axle which greatly improves traction in the front wheels. This is why your Fiestas and Fiats, usually front wheel drive cars, have the big advantage in the snow. You can improve the situation in rear wheel drive cars by putting sand bags, or something equally as heavy, in the boot (over the axes) but I can't say I've ever noticed a significant improvement doing this.
On the whole, rear wheel drive cars are pretty crap in the snow. My girl friend has an MX-5 (she doesn't get to drive the Lexus ;-) and has the same problem, being a rear wheel drive car. I generally avoid driving the car in the snow at all and stay at home. Fortunately, I have a very understanding boss.
To emphasise this point, I was at home last week (snowed in) and saw a BMW (also rear wheel drive) skidding all over the place and getting no where fast whilst a Renault Clio casually over took it. Power to the small cars!
Of course, tyre size also plays a part and, as already suggested, the thinner the tyre the better (... in the snow).