I know this legend for a long time now and believe it (like countless others, no doubt) without bothering to check whether it is true. Then I read an article on the Norway lemming (Summer 2009 edition of BBC Wildlife magazine), which exposed the origin of the myth.
According to the BBC Wildlife magazine:
The number one myth about lemmings, that they commit mass suicide by jumping off cliffs to drown themselves, is quite modern. Sometimes it even comes with a 'scientific' justification: the suicide would benefit the species in times of overcrowding. It seems so heroic, but unfortunately such behaviour simply does not exist in the animal kingdom. This did not stop Walt Disney from including footage of a lemming suicide in White Wilderness, released in 1958. The film crew bought about 1,000 lemmings from Inuit children, transported them to Alberta and created a set on top of cliffs by a stream. The lemmings were herded together on the edge - and driven over it. Needless to say, their 'fatal' plunge was a huge hit with the public.
So, always remember (especially if you have young impressionable kids like mine), don't belive everything you see at the movies.