Scott Bourne, the host of The Apple Phone Show, saved $5750 on a new car that he just purchased by using his iPhone during the negotiation. It looks like the most productive uses of the iPhone were recalling email correspondence that had occurred between him and the dealership, and verifying the information that the dealer's salesperson offered during the actual negotiation.
Kathleen and I bought our Honda Accord in December 2006 when we both had Treo 650s. At that time, we could recall our emails with the dealership as Scott Bourne did. However I'm guessing that we had less useful information in our emails than he did.
The web capability of the Treo 650 was primitive compared to my iPhone. If the dealer had open WiFi for use by its service customers (likely at least in California), the iPhone would be that much more useful.
Scott doesn't say what kind of car he bought. I assume that in order to get a $5750 discount on the price, he had to purchase a luxury car in the range of a Mercedes or a BMW.
I think what I learned from reading this article was that I should do some introductory email correspondence with dealers before ever going on a lot. If I had corresponded with dealers prior to going to any showroom, I bet I could have gotten the ask price to move in a favorable direction.Of course there's no question that I'll bring my iPhone when I shop for my next car.