If you haven't opened up your January 2008 issue of Art Education yet, you'll be happy to see that the instructional resource center pullout is: Eliot Porter and Robert Glenn Ketchum: A Conversation on Color, Composition, and Environmental Conservation. Although recommended for grades 5-7, it is certainly a great idea for higher grades.
In addition, there is a nice interactive site designed for student learning from the Amon Carter Museum (in Fort Worth, Texas) : http://www.cartermuseum.org/edu_guides/porter/
A discussion of the two photographers on outdoorphotography.com http://www.outdoorphotographer.com/content/2006/dec/form_function.shtml
Robert Glenn Ketchum's website: http://www.robertglennketchum.com
And the book: Regarding the Land: Robert Glenn Ketchum and the Legacy of Eliot Porter