I just spoke with a rep at mastercard and she confirmed coverage through priceline.
I have to admit though, that I see where ChaimB is coming from. The rental insurace agreements for Amex says as follows (excerpted):
How To Activate CoverageCoverage for theft of or damage to a Rental Auto is activated when the Cardmember:
1. presents his or her eligible Card to the Rental Company to reserve the Rental Auto, by making a reservation; or by placing a hold or deposit at the time the Rental Auto is checked out;
2. declines the full Collision Damage Waiver or similar option (CDW), or pays for a partial collision damage waiver, offered by the Rental Company;
3. is the primary renter, which is defined as the Cardmember, who is named on the written agreement with the Rental Company as the person renting and taking control and possession of the Rental Auto (“Primary Renter”); and
4. uses the Card to pay for the entire auto rental from the Rental Company at the time of vehicle return.
"Commercial Car Rental Company" or "Car Rental Company" means any commercial car rental agency which rents Rental Autos.2 For the purposes of this Description of Coverage, Commercial Car Rental company means "Rental Company."
I can see how this can be read to require you rent directly from the rental company, but I think this can also be read to include reservations through priceline.