"Open-source is nothing more than peer-reviewed science." --Marc Andreessen, inventor of the first Web browser, quoted in conversation by Thomas Friedman in The World is Flat. The question, then, is who are the peers? Friedman was writing about the development of the Apache web server, an extremely successful open-source application. The edits and additions to the code are reviewed by a board of programmers. And programmers at major institutions, and at IBM in particular, oversee the quality of the work to insure its continued viability. The CK-12 Foundation also has a review board, a "membrane" in Jaron Lanier's terms, so that you know that the textbook you are downloading has good information in it.
Open-source material depends on one critical element: intelligent participation. This applies to teaching too. We can use open-source materials like wikipedia in teaching if we have intelligent participation by intelligent teachers. Teachers have to know enough to spot a fake, sense an author's hidden agenda, cut through the spin or point out a glaring ommission. Teachers have to be authorities in their own right and to be able to discern authority in new media sources.