If there are lots of nice cars, it's a good sign that the company is doing well and compensates its staff accordingly.
If there are lots of older cars, it's probably a geek-friendly start-up atmosphere, not making much money yet, but everyone's in it together.
If there are a small number of very expensive cars in reserved spots for senior management, and in the few other spaces available there are lots of old cars, IME there is a 100% probability that I will not want to work there.
If people start doing things like car pooling, this almost perfect way to avoid wasting several hours in an interview for a job I'm never going to accept anyway will be spoiled, and I'll have to find some other obvious prejudice to use! :-)
What do you do? Get to the parking lot, look at it, and then just call them and say, "I've decided not to interview with you"?