Back in the late 1990s, I volunteered for a while with the CNIB helping a young woman finish off her high school diploma. (She was already in college, but they wouldn't let her graduate until she'd officially completed Grade 12.) The first course I helped her with was Family Studies, and I remember very clearly how I felt when Question 1 on the final exam began with the words, "Examine the graph in Figure 1..." This course was officially rated "Accessible to the Visually Impaired" by the Ministry of Education, but clearly, somebody hadn't actually tested the materials. As an answer to the question, she had me write, "I can't---I'm blind," but it still took four months of phone calls and letters to get someone to mark that exam out of 75 instead of out of 100.
Jump ahead to the fall of 2010. I had (literally) bumped into a computer science instructor at the University of Toronto who happens to be unsighted, and I mentioned the videos I'd been posting on the Software Carpentry site. He was teaching an introductory programming class; maybe the material would be useful to his students? He hemmed and hawed for a moment, then admitted that he'd actually visited the site, but hadn't been able to follow the lectures. Yes, there were full-text transcripts of what I was saying, but (and here he actually sounded apologetic) he wasn't deaf: he was blind. Whenever I referred to a diagram or mentioned "the highlighted bit of code", he lost the thread. Those diagrams and highlights only existed as pixels, not as some kind of markup that a screen reader or other tool could "render" for him.
Now, take a moment and go pretty much any online education site, like the Khan Academy or MIT's OpenCourseWare. Close your eyes, and see how much of the lecture you can follow. Some aren't bad—Prof. Wyn Kelley's American Literature course, for example, works almost as well on an iPod as it does with video. But every time an instructor actually relies on a diagram to explain something, everyone student who's visually impaired loses a couple of marks, either because they can't follow at all, or because the extra time it take sthem to figure it out is time their sighted peers are spending mastering the next topic. Things like the in-browser "code and sketch" tool that John Resig is building for the Khan Academy may look very cool, but what happens to people who can't look? It's no good saying, "Let us figure out how to teach online, then we'll worry about special needs," because experience shows that adding accessibility after the fact works just about as well as adding security after the fact (i.e., it doesn't).
Ed-tech's advocates keep saying that they want to make the best teaching accessible to everyone. Respectfully, they're not doing that, and as schools rush to get students online in order to save money, people who are already disadvantaged will be left even further behind. I don't have any answers—I don't think there are any easy ones—but those of us who can see shouldn't lose sight of the needs of those who can't.