Okay, I found it. It's in
Uncle John's Legendary Lost Bathroom Reader, page 166 "The Truth About Lemmings". Here are some excerpts of the article:
"In 1958 Walt Disney produced
White Wilderness, a documentary about life in the Arctic."
"They quite literally eat themselves out of house and home," says the narrator. "With things as crowded as this, someone has to make room for somebody somehow. And so, Nature herself takes a hand...A kind of compulsion seizes each tiny rodent and, carried along by an unreasoning hysteria, each falls into step for a march that will take them to a strange destiny."
"The last shot," says critic William Poundstone, "shows the sea awash with dying lemmings."
The narrator says: "Gradually strength wanes...determination ebbs away...and the Arctic Sea is dotted with tiny bobbing bodies."
The Truth:
"...the lemmings were brought to Alberta - a landlocked province that isn't their natural habitat - where Disney folks put them on a giant turntable piled with snow to film the "migration segment."
"Then...they recaptured the lemmings and took them to a cliff over a river. "When the well-adjusted lemmings wouldn't jump," writes Poundstone, "the Disney people gave Nature a hand [and tossed them off]...Lemmings don't commit mass suicide. As far as zoologists can tell, it's a myth"
More info at snopes (awesome Urban Legends website):
http://www.snopes2.com/disney/films/lemmings.htm
Dan East