I did a little web research too...
According to the Duck Tape Guys:
"Is it Duct or Duck? We don’t want you to be confused, so we will explain. The first name for Duct Tape was DUCK. During World War II the U.S. Military needed a waterproof tape to keep the moisture out of ammunition cases. So, they enlisted the Johnson and Johnson Permacel Division to manufacture the tape. Because it was waterproof, everyone referred to it as “duck” tape (like water off a duck’s back)."
Here's the web site:
http://www.octanecreative.com/ducttape/duckvsduct.html
I'm not nearly old enough to remember any of this but that Duck/t Tape was invented to seal ammunition cases in WWII. Maybe Manco hit the Duck tape scene after it was already being called Duct Tape?
Who knows, maybe these guys are quoting the History Channel?
Here's a site from Henkel, who appears to own Manco, that seems to support both sides:
http://www.henkelpr.com/doc_text.asp...t=ppr&cls=DUCK
I'll still vote for Duck but I'd say both are right.