There is a current discussion along this thread on the Independent Schools Network which cites an article that was published in the Times: "In a Digital Future, Textbooks are History".
I love validation I must say and this was one of those times. Our school has not used textbooks for most courses for at least five years. Part of it was a cost factor combined with the amount of differentiation we do on a daily basis with multi-age classes for students with learning disabilities. Another reason was that there is rarely one textbook that includes what we want for all units in a good format. We determine the content and then supplement and with the advancement in technologies, we use quite a bit of digital text. Continuing to grow in this fashion, we are also finding that by posting links to this "text" on our wikispace, etc.. we are also negating the "I left it at school/home" factor and use an on-screen reader for those who need support that way. It's been quite powerful really!