I assume you have subs in your trunk? If so, even though the trunk is pretty well insulated, I believe Dynamat-ing the trunk lid and maybe the trunk walls (although they'll be a pain in the butt because of all the wires/contours), you'll get even a better bass sound, plus lessen the rattling.
I'm about to do mine, so try to remember to post results. Also, by the way, I saw a Dynamat trunk kit at Circuit City today for about $100.
Better picture coming!