I recently read a fascinating survey of 1000 blind and visually impaired internet users. It's a long article but is largely graphs and data tables – so is easily consumed. If you want the short version, check out the comments and conclusion.
The most surprising parts for me were;
- Websites that auto-play music (MySpace most notably) are extremely difficult because the sound makes it hard for the user to listen to screen readers. Further, the flash music player is usually inaccessible for screen reader users, so the music can not be stopped and the page is completely unusable.
- Headings make website navigation significantly easier for all screen reader users.
- The order of content and navigation in the source code is not particularly important.
- Skip-links are useful but are ignored by a significant number of users and the rest seem to get by okay without them.
- In general screen reader users hate Facebook, missing or meaning-less
alt=""atributes, and "web sites which refresh on the fly" (I think this refers to AJAX/AHAH functionality, but I'm not sure).
A smaller, less quantitive and more qualitative Australian usability study of website accessibility confirms many of these findings. It's arguably flawed and out-dated but interesting nevertheless.
Finally, a debate on Planet Drupal about a related topic, content ordering in the Zen theme and reasoning for it, is what led me to these.