I vote dual displays, I like having two distinct workspaces, it kicks arse for:
-programming (originally usage)
-writing papers (sources on one side, paper on the other)
-making powerpoints (see above)
-chatting online/reading email/itunes open all without disturbing primary workspace
seems to be too much jugglign with windows if just one large display
OTOH, one big display would be better for:
-movie watching
-gaming (maybe, I'm not much of a gamer) - you would need a kickarse graphics card to drive a humongous display at full blast