Of all the major presidential candidates running for the office of President of the United States in 2008, Barack Obama is the only one, in either party, to explicitly and strongly endorse separation of church and state.

You may be aware that Obama does appeal to religious faith and that he does speaking openly about his own religious faith, but he also clearly understands the difference between personal belief and government policy. Having said that, Barack Obama does certainly endorse religion in general and uphold religion as a valuable part of what he believes is the fabric of society.

Here is what Barack Obama has said about separation of church and state:

I went to a Catholic school in a Muslim country, so I was studying the Bible and catechisms by day, and, at night, you'd hear the [Muslim] prayer call. My mother was a deeply spiritual person. Her view always was that underlying these religions was a common set of beliefs about how you treat other people and how you aspire to act, not just for yourself, but also for the greater good. I am a follower, as well, of our civic religion. I'm a big believer in the separation of church and state. I am a big believer in our constitutional structure. I'm a law professor at the University of Chicago teaching constitutional law.
Chicago Sun-Tim, "I have a deep faith", 2004

For one, the separation of church and state in America has preserved not only our democracy but also the robustness of our religious practice. After all, during our founding, it was not the civil libertarians who were the most effective champions of this separation; it was the persecuted religious minorities concerned that any state-sponsored religion might hinder their ability to practice their faith.

This separation is critical to our form of government because in the end, democracy demands that the religiously motivated translate their concerns into universal, rather than religion-specific, values. It requires that their proposals be subject to argument, and amenable to reason. If I am opposed to abortion for religious reasons but seek to pass a law banning the practice, I cannot simply point to the teachings of my church. I have to explain why abortion violates some principle that is accessible to people of all faiths, including those with no faith at all.
 
This might be difficult for those who believe in the inerrancy of the Bible, but in a pluralistic democracy, we have no choice. Politics involves compromise, the art of the possible. But religion does not allow for compromise. To base one's life on such uncompromising commitments may be sublime; to base our policymaking on them would be dangerous.
Politics need not abandon religion, 2006

Obama's positions on religion are complicated though, and he has done a tremendous job of simultaneously appealing to religious voters while strongly supporting separation of church and state. Indeed some could argue that his open endorsement of religion in general affords him the ability to speaking in favor of secular government. Obama's views on religion and separation of church and state are no doubt, as he himself has said, highly influenced by his diverse background and upbringing. He comes from a mixed Christian and Muslim background and his mother happens not to have been a member of any particular religious group. He notes that his mother was very "spiritual" and felt that all religions shared some core elements of truth. Obama was, of course, also a law student at Harvard. With this diverse background it is no wonder that Barack Obama supports separation of church and state, and he appears to have done a good job of walking the fine line between personal belief and public policy making.

While Barack Obama does clearly speak to the religious interests of voters, and does in some cases tout the role of religion in benefiting society, he also speaks more strongly, and more directly, to the issue of separation of church and state than any of the other presidential candidates.