SharePoint 2010 Master pages and layouts are now WCAG 2.0 AA compliant, using div tags and producing XHTML.
But, don't assume that this means any site you create will also be compliant to AA standards. A site is only a collection of pages, and compliance is on the page level. Sadly, with SharePoint WCM and page editing it is possible to insert non-complaint and down right ugly code in to your SharePoint sites. I was just on a 2010 site and looking at the HTML I was happy to see that the tap and quick navigation were all managed by div tags. I also so a div tag for the main body, but inside this main div tag the developers had dropped a mass of HTML which included tables inside of tables inside of tables. The page as a whole was not proper XHTML and not WCAG 2.0 AA or even A compliant.
When implementing a SharePoint site, especially a WCM section it is critical to put some review processes in place to insure that the content being created is as complaint as the OTB master pages. Don't assume that just having SharePoint 2010 will get you to AA compliance, you need to have complaint content creation as well.