Go to Abbey Newsletter, Vol. 21 no. 4, Nov. 1997. "How Stable are Photos on RC Paper" by Douglas Nishimura, IPI. This is pretty similar to his post on the Distlist, but contains some more info about ANSI recommendations for storage and a little more about the polyethylene base.
Like I said, read this stuff--there's a ton of material on CoOL and links to dozens of other sites as well. KT