Saylor.org is offering prizes to open source undergraduate textbook authors. It's under a program called The Open Textbook Challenge. While this is a great step in the right direction, more needs to be done to develop assessment and feedback tools for authors and adopters. Nice job Saylor!
(Update Mon Feb 13 09:21:47 PST 2012: On closer read, this reward program is only for authors with copyright on existing textbooks who open the texts under a creative commons license. Still a good program, but not quite the incentive for open-source authors that I initially thought.)