I remember when "document-centric" was all the rage in UIs (often went hand-in-hand with the term "object-oriented"... right around Windows 95/OpenDoc/Workplace Shell timeframe) and I would think that it seemed odd, because most users I knew had nearly no real concept of document/folder hierarchies.
You'd ask a user where their document is and they'd answer — just like they do today — "It's in Word."
Ubuntu decided to go that way for some reason, but I think that vanilla Gnome tries to continue on the document-oriented path. gnome-shell searches documents by default, includes Beagle (or whatever the native tool was called) and lately the Zeitgeist project. We'll see what they plan to do with Unity...
