On June 24, 2012, local time, Stuxnet stopped replicating. Its code still runs, but it doesn’t propagate via USB sticks any more.
Now that’s a very unusual feature. Why would it do this? Government-style thinking?
Its later cousins, Duqu and Flame, act differently, with a 30-day lifespan default in one case, and a self-deleting “suicide” feature in the other. Is this a pretty clearly developing line of thinking, do you think? That’s probably too simplistic, though.
See http://gcn.com/articles/2012/06/26/stuxnet-demise-expiration-date.aspx