stevenberlinjohnson.com: Leopard First Impressions: The Time Machine

I suspect this is a bit generous, but it's related...  Also see Johnson on "Why the London Plot Helps the Democrats (or at least, why it should help them)" 

As far as I know, this is the first mainstream OS to adopt David Gelernter's "lifestreams" metaphor for representing changes in data over time. Essentially, it deals with time the way the physicists did: it makes time a new dimension, extending out "behind" the screen. The normal 2D representation of, say, the contents of a given folder represent its "live," real-time state. When you want to go back in time -- to, say, retrieve a deleted file -- past versions of the folder appear as cards stacked behind the current version that you flip through.

Source: stevenberlinjohnson.com: Leopard First Impressions: The Time Machine