Here are links to an interesting 2-part article from the San Francisco Chronicle about the environmental impact of pets.

The 1st part is about the impact itself; the 2nd part is about what we can do.

http://www.sfgate.com/cgi-bin/
article.cgi?f=/g/a/2007/11/13/petscol.DTL

http://www.sfgate.com/cgi-bin/
article.cgi?f=/g/a/2007/11/27/petscol.DTL