The February 2011 special issue of Game Studies, the free web-published international journal of computer game research, focuses on the subject of game reward systems. Four articles in particular may be more interesting than others, depending on your perspective, but all of them are certainly worth a read.
Deakin University's Christopher Moore analyses how hats, achievements, and other player-centric rewards have impacted affective relationships in Team Fortress 2. Mikael Jacobsson, associate professor at Malmö University, takes an in-depth look at Xbox 360 achievements and the culture of achievements surrounding it.
Douglas Wilson, co-designer of IndieCade 2010 finalist B.U.T.T.O.N. and PhD candidate at the IT University of Copenhagen’s Center for Computer Games Research, takes a different perspective on player rewards in an insightful piece on the game's design. And if you think you're up to it, the University of Bedfordshire's Alison Gazzard's more theoretical look at reward systems is worth the effort.
Go forth and be educated!