Mark Lepper has done a lot of research on intrinsic motivation, including the classic studies showing that external rewards can undermine intrinsic motivation. You can see what looks most relevant in this list of his papers, or search Google scholar with those keywords (Lepper intrinsic motivation).
There is also some recent research on choking, I believe by Dan Ariely, which shows that people do some tasks worse with large incentives than with small incentives because of the pressure to succeed.
This Wikipedia article on the overjustification effect has a number of potentially useful references.
I've been thinking about incorporating my Vanity and Ambition in Mathematics into a top level posting. If possible I would like to situation my remarks and the quotations that I cite with respect to the existing experimental psychology literature. When I've discussed the material in the aforementioned article with people in psychology they've sometimes made reference to recent findings that monetary incentives reduce performance on certain kinds of tasks, perhaps suggesting that intrinsic rather than extrinsic motivation is key for performance on certain kinds of tasks.
I'll do my own research, but does anybody know of any relevant studies?