I tested demo.sparkdrupal.com alpha5 with JAWS13 / FF14, NVDA2012.2.1/FF14 and VoiceOver/Safari on Lion. None of these combinations could edit the contenteditable region of the DOM. JAWS and NVDA after intering forms (interative) mode could not even read the content of the region.
Some other WYSIWYGs like CKEditor and TinyMCE use contenteditable, but in the background, while editing is still in an iframe. The accessibility of these editors is improved. Using a screen-reader you can read and edit the content, even if you don't get a live sense of the semantics (list/headings/etc) applied to any portion of the content.