A lot of your accessibility is in (roughly) 4 areas, the design, the navigation of the site, the content and the "semanticness" of the markup (is that a new word?). With most CMSes that are worth their salt, you should be able to control all 4 without too much hassle.
Are you currently familiar with any CMS at all? If not, then the best thing to do would be to find the best CMS for what your skills are and researching for the "best in class" as it were. One of the important criteria would of course be how much control you have over the code that the CMS outputs, as this will greatly affect how much accessibility you're able to sprinkle around your markup.
There is a good list of CMS packages here in the Content Management forum http://www.sitepoint.com/forums/showthread.php?719241-What-CMS-packages-are-available
I've had good experiences with Wordpress in terms of how much control you have over the code. Even though some of the markup you can't control in a straightforward manner, often that doesn't matter too much because it will generate appropriate markup anyway. (For example, a menu might be generated as an unordered list.)
I've had a play with Concrete5 before and didn't find it too bad to work with (and AFAIK gives you 100% control over the markup)
If you're after a .NET based CMS, I've heard good things about Umbraco.