cK12 (http://www.ck12.org/) and Flat World Knowledge (http://www.flatworldknowledge.com/) are two companies trying to turn textbook publishing on its head. cK12 is a non-profit while Flat World Knowledge will be (Spring 2009) attempting to make some money in this endeavor.
Classroom 2.0 has a good article about them. Disrupting Textbooks: cK12 and Flat World Knowledge (http://www.classroom20.com/profiles/blogs/disrupting-textbooks-ck12-and)
vK12 is online now, though I found that their website has some bugs in it. Also, I haven't looked to see how well peer-reviewed their texts are.
Flat World Knowledge is looking to "go commercial" in spring 2009.