I don't pretend to know whether Obama is Christian, Muslim, or anything else. I do find it odd that he would cancel the White House's National Day of Prayer breakfast but hold a dinner to celebrate Ramadan, but neither of those things are germane to what I really want to discuss, and that is whether prayer is a sign that someone is a Christian. And, of course, the short - and Biblical - answer is "no."
It's probably helpful for purposes of this discussion to define prayer, and I take a very simplistic view: prayer is speaking with God. We can make it complicated or ritualistic, but conversing with the Deity is the essence of prayer.
Prayer is an important characteristic of the Christian faith. Jesus Christ taught His disciples how to pray, and He spent considerable time in prayer. The Christian who doesn't pray is missing a cornerstone of his faith.
But, guess what? Using my broad definition of prayer, the practice is not limited to Christians. In fact, Satan and his demons pray. A passage in the book of Revelation describes Satan's habit of coming before God to accuse us of sin, continually (as if God didn't already know these things!). And one of Jesus' earliest recorded miracles was the exorcism of an evil spirit from a man; Scripture records a short, desperate prayer by said spirit before it was evicted.
These simple examples help demonstrate that it's not what we do that makes us Christians; it's what we believe. In this regard, the President's spokesman did him no favors in attempting to describe his boss's faith.
Two additional thoughts. First, while one might argue that the faith, or lack thereof, of an American president is nobody's business, it's a fact of life that such things are still of interest to an apparent majority of Americans. I fear for our nation when that ceases to be.
Second, while we're in the neighborhood, just as praying isn't necessarily an indication that one is a Christian, neither is knowing who Jesus Christ is. Knowing Christ is not the same as accepting Him as Lord and Savior. Check out this passage in Acts where more evil spirits make this point.

