When Obama was sworn in on Sunday, he requested that Justice John Roberts end the oath with "so help me God." The phrase is optional - it isn't required as part of the president's swearing in, but it's been used by every president since 1881.
Atheists have battled against the phrase, claiming that it violates the constitutional ban on government "endorsements" of religion. Activist Michael Newdow of California brought a challenge against the phrase that made it as far as the Supreme Court, who declined to hear the case in 2011.
There's a myth that "so help me God" dates back to George Washington's oath of office, but it's been debunked by historians at the Library of Congress.
However, there's always been a lot of mention made of God at inaugurations. George Washington's inauguration speech "began with 'fervent supplications' to the 'Almighty Being' and concluded by seeing God's 'divine blessing' for the nation, writes Cathy Lynn Grossman at Religion News Service.