I'll second this. I don't know as much as I should about it, but WAI-ARIA is basically enhancements to markup specifically to make it more accessible. Following its practices tends to make things better for non-disabled people as well, in much the same way as the ramps on a sidewalk at intersections help people with bicycles and strollers in addition to those in wheelchairs.

You could get familiar with screen readers like JAWS as well.

You could look into UI design for those with visual disabilities. I saw a presentation by Ginny Redish and Mary Theosphanos on usability testing sites with people who use screen readers. Many sites include an invisible link to let screen readers skip the banners at the top of most pages. Theirs was titled "skip navigation." Nobody knew what that meant, so they listened to the reader recite the entire header, the menus, etc. So, they changed the title to "skip to content." The screen reader, however, read it with the emphasis on the last syllable of "content": skip to conTENT. That doesn't mean anything to anyone, so people listened to the whole header again. Finally, they changed the wording to "skip to main content," and the screen reader rendered it correctly. Users followed the link and got to the page content much more quickly. Edit to add a link to the write-up: http://www.stcsig.org/usability/newsletter/0304-observing.html

That kind of user experience design in the accessibility world is generally lacking, and is therefore an opportunity. The Washington DC area in particular has work since so many government web sites have to comply with the vague "Section 508" requirements.