Time provides some reasons why both Obama and McCain steered clear of Godtalk in their debates:
The personal reasons may, in fact, be more important than the larger political context. Neither Joe Biden nor Sarah Palin were so restrained in invoking the deity. “God” was mentioned seven times, five by Biden, twice by Palin. And in some ways, both running mates have been carrying the religion ball for their candidates since they were chosen in August.
More here.