Black Sabbath Reunion is on

Black Sabbath are getting together again for Ozzfest this summer but why isn’t Bill Ward involved? Anybody know?
UPDATED Bill Ward provides his reasons in a letter quoted on I Ate Your Microphone. Sounds like contract stuff reminiscent of the whole Peter Criss fiasco with Kiss.
UPDATE#2 According to Bill Ward’s official site he may now very well be part of the reunion after all.