Katherine Lynch gives an overview of her presentation on Accessibility in Drupal 6 and Drupal 7: Contribute accessible modules and themes. Just as internationalization is abbreviated i18n, and localization is abbreviated l10n, accessibility is abbreviated as a11y.
She also provides from tips for how to test your site through the open source screen reader called Thunder or the demo version of the most popular screen reader of JAWS. Lynch briefly describes how a screen reader will parse and communicate the content on a web page.
Lynch also recommends using a couple of FireFox extensions of the Firefox Accessibility Extension and Fangs, which will help you verify the accessibility of your site. In her DrupalCon presentation slides, she also mentions VisCheck to see your site as a colorblind person would perceive it.
Finally, she provides some tips for how to create equivalent information to supplement rich media, and how HTML5 may help with accessibility in the future.