AMEX' lays it out in their Operating Procedures for U.S. Merchants under Section 8, Special Industry-Specific Procedures:

8.4 Motor Vehicle Sales

"We will accept Charges for the down payment or the entire purchase price of new and used motor vehicles only if:

• [*]
• [*]

If the Cardmember denies making or authorizing the Charge and you have not transferred title or physical possession of the motor vehicle to the Cardmember, we will have Chargeback rights for such Charge."

Oh, and for the sake of staying on topic, last February I bought a Chrysler 300C (with Hemi!) and put $5K down on my Visa. Got 10K miles, plus a 10K miles bonus, plus 2.5K "Citi Dollars" (Not sure if $5K was a charge limit or not... that's all I was going to charge anyway).