A common misconception among web developers is that screen readers only read the non-JavaScript page. Due to this misconception, we sometimes assume it's unnecessary to make JavaScript apps and other functionality accessible. This is categorically false.
According to a May 2012 survey conducted by WebAIM, 98.6% of screen reader users have JavaScript enabled.
Be sure to make efforts to ensure that the JavaScript functionality (apps, custom UI controls, forms, etc) within your site is accessible. It could be as simple as opting for native links and form inputs instead of custom scripted form elements.
Also be sure your website is accessible for the ~2% of users (globally) with Javascript disabled.
Great point. You may want to expand on the conclusion to include some specific examples or resources. For instance, recommend fallback on the browser's default link and input behaviors instead of relying on javascript click handlers. (per http://www.w3.org/TR/WCAG20-TECHS/failures.html#F42)
WCAG has a list of client-side scripting techniques that you can point to:
http://www.w3.org/TR/WCAG20-TECHS/client-side-script.html
also:
s/insure/ensure
s/it is assumed/we sometimes assume