April 9, 2012
TV Line says that "according to an insider" Dana is confirmed to be reprising her role as Katherine in the Desperate Housewives series finale that will air May 13.
This story also states that Julie Benz will not be reappearing as Robin.
Dana recently tweeted that news about this was forthcoming, so even though she has not subsequently confirmed, it does seem likely to be true.
See the story at tvline.com