WPF 508 Compliance
There is this white paper at WindowsClient.net that appears to contain guidelines.
There is also this pdf about accesibility in Infragistics' WPF controls. It may contain some clues, but it is in the context of custom controls rather than entire UIs.
I find Engineering Software for Accessibility portal page very useful, as it covers all aspects of the accessible application development - with a handful of links to MSDN pages.