Update: I have finally fixed my sub rattle. I purchased 2 sheets of dynamat extreme and 3 sheets of dynamat original from eBay for about $85.
--I pulled the interior quarter panel cover....fairly straightforward, except for some binding at the Curtain Air Bag cover. Those with RES, be sure to pull the A/V connector at the back of panel. I applied the the full 2 sheets of extreme against the sheet metal body of the car. And I had some left over and covered the inner sheet metal, and when I ran out of the good stuff, I switched to the dynamat original.
--Results: In a word "awesome". I am very happy. When you tap your knuckle to the outside quarter panel, it sounds like the metal it more than 1/4" thick!
--The subwoofer sound from factory: Seriously lacking. I heard rattles on the day I purchased it.
--The "dealership fix": My dealership installed a 12"x12" "pillow" directly behind the sub against the quarter panel sheet metal. There were noticeably less rattles, but also noticeably less bass. A buddy of mine said, "...it doesn't even sound like there is a sub installed."
--My fix: Ditch the pillow, install dynamat extreme! I have bass now. Not Earth shattering bass mind you.....but I can listen to Eminem's new CD and hear almost all of the bass drops. Now there are no rattles outside, and none inside when the bass it hitting.
Sorry for the lengthy post, but I am pretty excited, and thought I'd share my dynomat experience. One day I may do my whole MDX!