They did fairly major backend-user-interface testing, polling, and managed improvements for WP 2.7. I’m pretty sure accessibility was one of their criteria but didn’t seem to be the #1 guiding principle. See
http://wordpress.org/development/category/ui/
Do you find the Dashboard to be less than accessible? What flavors of accessibility are you looking for?
Some things can be accomplished with plugins, or Admin color schemes. For example if you need higher contrast or larger fonts.
Basically I’m setting up WP to act as a kind of CMS and it need to be used by people who use screenreaders and the like. I’m fine myself, as I don’t have any issues. I know someone who is a fair expert on making accessible sites so I might get her to cast her eye over it to see how it shapes up (and report any comments back here)
I’ll take a look at that link, thank you!