I did that through email, and it kind of works with one additional question. "Do you actually have the car?". I sent emails that essentially said I am buying a car tomorrow, this is the car I want and the options and color, I will show up with cash tomorrow night, whats the best out the door price.
The problem was that most dealerships quoted me a price (the lowest two were quite a bit lower), but when I showed up with cash, they didn't have the car and couldn't get it. I am not sure how inventory works and if there was actually a car when I asked about it, or if it never really existed, but the dealership I showed up at said it couldn't find the car I wanted anywhere. When I called the other dealerships, none of them could get me the car either. This was during the last days of cash for clunkers, so base model cars were hard to come by.
For me, it was a good idea. I knew what I wanted and wasn't worried about haggling for every single penny. I just wanted to walk in, pay my money and take the car. Maybe I would have paid a little bit more than I had to, but it saved all the time of driving to each dealership and trying to work a deal 10 times.