You can buy a Navigo Decouverte card without being a resident or having a French address.
Please see my article here on
BonjourParis.com
http://www.bonjourparis.com/story/navigo-natives-through-paris-metro/?sid=27698
or for a version with some updated comments, at
http://www.writingtravel.com/2009/01/navigo-pass-for-metro-now-a-must-for-bargainseeking-paris-travelers.html
Apparently, the Navigo Decouverte was not designed for tourists. That's why the Paris tourist board promotes the more expensive Paris Visite pass.
Navigo Decouverte is actually meant to address the privacy concerns inherent in the Pass Navigo, which apparently can track a traveler's movements through the system.
All you need to do to get a Navigo Decouverte is ask for one at a full-service tabac or in one of the larger Metro stations. Pay 5 euros for the card plus 17,20 for the current week ((no partial weeks allowed). Your card is good forever (or until they come up with another gimmick).
You'll have to get a postage-stamp sized photo but you can use the pass as-is until you do so. There are photo kiosks in Metro stations; I think I got my photos at Monoprix.