I'd like to chime in and point out that Drupal has put a ton of work into making their product accessible out of the box. You may wish to consult their accessibility statement; and they have a very active Drupal accessibility discussion group, which may also be of interest.
That said, I'd just like to add that regardless of which CMS you use, accessibility is more cultural than technical. It does not matter whether your CMS starts accessible if it does not stay that way. Anybody who touches code for the web needs to be trained on what accessibility means, what works, and what doesn't. They don't necessarily need to become experts, but if you do not attend to the training side of the equation, you're very likely to wind up with poorly coded content that breaks accessibility because the writer simply never stopped to consider how well it works for people who can't see, or hear, or who are paralyzed, and so on.
I did an accessibility review of a site for a large library once, in which I came across this piece of code:
<!--don't know why "hiddenNav" is here - rh 3/21/08
<img src="/exhibitions/web/woodstein/images/spcr.gif" border="0" alt="Go to the Top" />
This is one of the saddest things I've ever seen in the world of coding. At some point, the site had a coder who know how screen readers worked. The "hidden nav" was put there to provide a convenient method for screen reader users to return their cursor to the top of the section. But the institution failed to internalize the practice of accessibility, and after that knowledgeable coder left, their successor disabled this accessibility feature -- not out of malice, but out of puzzlement. "RH" had certainly never used a screen reader, if indeed they had heard tell of such a thing, and the code really doesn't make sense unless you realize that it's supposed to be read aloud.
So -- I applaud your efforts to pick an accessible CMS. But please, please don't imagine that the job stops there. If you neglect the human side of the equation, your good work will slowly but surely decay over time.