I know we've talked about how rewards affect student motivation many times before, but this article in the American Educator approaches the question "should learning be its own reward?" from the perspective of cognitive science. It is probably the most comprehensive article about using rewards to motivate students I've read so far, so if this is something you're interested in, I highly recommend that you check it out. Here's an excerpt:
Concrete rewards can motivate students to attend class, to behave well, or to produce better work. But if you are not careful in choosing what you reward, they can prompt students to produce shoddy work--and worse, they can cause students to actually like school subjects less. The important guidelines are these: Don't use rewards unless you have to, use rewards for a specific reason, and use them for a limited time.
He goes on to address the question from three different angles: Is a rewards system immoral? Does it condition kids to have unrealistic expectations? And does it actually decrease motivation?
What he talks about reminds me of my experiences teaching a study skills class in college. It was a 10-week course, taught by peer instructors, that all students on academic probation were required to take in order to stay in school. Each week I, and a fellow peer instructor, went over a different skill that could help the students improve their grades--time management, goal setting, etc. Sometimes the lessons were pretty dry, so we decided to reward our class with treats at the end of the day if they paid attention and participated in discussion. What we found was that our students came to expect the reward, and if it wasn't promised to them at the beginning of class, they were much less likely to take the class seriously. And these were college students! By the end of the course, I wished we had never started rewarding them in the first place.