http://www.ted.com/talks/dan_pink_on_motivation.html
I'm actually half-way through a book on a very related topic (competition), which has the same conclusions as this talk. Basically: external motivators (like incentives, bonuses, competition, etc) fail miserably when put beside intrinsic motivators (the drive to do things for their own sake, because they matter), except in purely mechanical tasks.
Really great talk, I highly recommend it.