I have recently been using WordPress heavily on the University of Illinois main website. We took many step when implementing WordPress to identify a theme framework that made webpages as accessible as possible. This eventually led us to the Thematic framework–a barebones parent/child theme which provides pretty good accessibility out of the box.
However, there are several accessibility issues that seem to derive from the WP core rather than themes. That’s way I was so happy today when I stumbled on this post regarding a WP accessibility plugin: http://chronicle.com/blogs/profhacker/make-your-wordpress-site-more-accessible/54631?cid=pm&utm_source=pm&utm_medium=en.
Updated Feb 17, 2014: here is another post about WP Accessibility: http://chronicle.com/blogs/profhacker/accessibility-ready-wordpress-themes/55683