Kim White, my colleague at The Institute for the Future of the Book, just posted on our blog a small informal case study of the "collaboration of the month" textbook featured at Wikibooks -- a spin-off project of Wikipedia developing open-content textbooks. Worth taking a look.
UPDATE: On fostering a better, more nuanced debate about Wikipedia and the open source model for education in general... HERE
(We're keeping Wikibooks in the corner of our eye as we continue developing our own digital textbook project next\text.)