Yeah Buzz, this happened in 1992 while Black Sabbath was touring for the album "Dehumanizer". Ozzy was doing his No More Tours tour, saying that he was going to retire from touring (remember that shit?)
Well, in one of Ozzy's/Sharon's first moneygrubbing whore moves, they decided it would be cool if Sabbath OPENED for Ozzy on two shows in California. Tony Iommi and Geezer Butler were all for this cash-enhancing move, so all they needed was to convince Ronnie James Dio.
So Dio, being the man that he is, decided that he wasn't going to bite the pillow and open up for the man that he has been compared to for his whole career, and he boycotted singing for those shows. This soon led to the demise (once again) of the Dio-fronted Sabbath.
This is how Rob Halford was asked to sing for Sabbath the first time.
That said, I have no idea how/why Halford sung for them again recently. I'll have to look it up.