It wasn't the Amex Platinum cover I was referring to, it was the Amex Premium Annual Travel Insurance. This is a very comprehensive travel policy (including car hire in North America) which is available to anyone, even if they are not Amex cardholders. This policy is expensive, but if your travel arrangements include a 'free' car rental it might work out cheaper for you to pay a little bit more for the Amex travel policy and not have to pay the outrageous charges for collision damage waiver and supplementary liability insurance which is necessary when you hire a car.
There are alternatives to the Amex policy - Natwest do something similar but you have to be a Natwest (Gold?) cardholder to take advantage of this. For Jane and I the Amex travel insurance policy works out the best. We always book flydrives, so rather than spend £22 per day on insurance for anything up to 28 days (2 holidays) costing £616, we pay £280 for our travel insurance and pocket the difference.
An appropriate disclaimer would be YMMV (your mileage may vary!).
Regards
Rob