from the "you can get anything you want on the web (but not necessarily what you need)" dept.:
http://www.octanecreative.com/ducttape/duckvsduct.html

p.s. - they have a really nice disclaimer: Note: To be legally called “Duct Tape” the tape must meet or exceed certain heat requirements. Some Duck® brand Tapes do meet this classification and will tell you so on their label.