Folk lore has it that Iommi brought in TM in case Dio jumped again.
Actually there's a non official site, Black-Sabbath.com, that has a fantastic timeline page detailing in excruciating detail every single lineup change. It seems that Dio DID jump ship, Tony Martin was really brought in not as a just-in-case basis, but specifically 'cause Dio didn't work out. Only when the label put their feet down both parties (Sabbath / Dio) decided to give it a go.
It's incredible how much Dio and Sabbath had a love / hate relationship. He came, he saved the band from near dissolution, then after two awesome albums their egos got the better of them, and the straw that broke the camel's back was the infamous sneaky mixing of Live Evil, where apparently Dio was changing the mixes to his favor in the night and Iommi was doing the same in the morning (or the other way around).
A decade goes by, Dio finally rejoins Sabbath, the thing falls apart even before the album is released only to be patched back together, and after one album Ozzy "retires" and the band folds AGAIN (tanks btw to Rob Halford to step in and creating two legendary, unique and heavily bootlegged nights).
It's only in 2006 that finally the guys get together for a third time, we get an amazing tour based only on Dio material, a new heavy, doomish and uncompromising album, and then Dio gets ill and dies.
I am so, so, SO glad to have seen Dio and Iommi together in Black Sabbath, twice. The other dream will remain one - Dio and Blackmore back together, gosh what a marvel it could have been (especially if Cozy Powell didn't die - I believe there were some talks in 1998 but Cozy died and the thing fell apart, with Blackmore going medieval instead).