According to Gallup, John McCain trails Barack Obama by 25 points among voters for whom religion is not "an important part of [their] daily life". McCain leads by 5 points among those who answer that question in the affirmative.
These sorts of numbers are generally described as a problem for the Democratic candidate. However, as Ruy Teixeira pointed out four years ago, if you had to pick a sign of this divide to be on, it might be on the side of the secular. That is because by almost all indicators, religious participation in the United States is decreasing. According to a Pew poll, 45 percent of Americans now completely agree with the statement that "prayer is an important part of my daily life", down from a peak of 55 percent in 1999. (There does appear to have a bit of a "God Bounce"/mini-revival in the mid-late 1990s -- not so much in the number of religious Americans, but in the activity and enthusiasm of those that do practice).
Moreover, the younger generation is less religious than the older generation. 19 percent of those born after 1977 say they are atheist or agnostic, as compared with 11 percent of Boomers (born 1946-1964), and 5 perecnt of pre-Boomers (born before 1946).
Barack Obama, of course, does need to at least hold his own among actively religious voters, who constitute 65 percent of the electorate according to Gallup. He is able to do so thanks to substantial support from African-American and Latino voters, while trailing McCain by 25 points among actively religious, non-Hispanic whites. Nevertheless, if these generational trends hold, then each year a coalition based on actively religious voters will become marginally less successful.