I like the look of SIteInifinity by Telerik as a content management system. Unfortunately it is not WCAG 2.0 compliant.
Are there any third party CMS systems that are WCAG 2.0 complaint?
There are two sides to your question: the main website as seen by users should be compliant with WCAG 2.0 and the backoffice as used by content producers and administrators should be compliant with ATAG (Authoring Tool Accessibility Guidelines)
A few points:
A lot of CMS which allow template customization can manage a website that conforms to WCAG 2.0 Level A or AA and many aspects or AAA. I work mainly with LAMP CMS so I don't know at all this other world ruled by ASP.NET ;) (a big STOP is adding a form element as a direct child of the body element, for no reason. Good luck with the rest of your code)
No CMS with untrained content producers can keep this level of accessibility for long
Few CMS are accessible to their content producers (the Authoring Tool part), and ARIA will help a lot (in a few years)