Ben Harper sings a beautifully poingant song, called "When She Believes" that would be perfect for a first dance. I wish I could find a link, so that you could listen to this amazing song, start to finish, but all I can find is the amazon.com link. But at least you can preview a little bit of it.
At a more informal affair it might even work as a walking down the aisle song. It's just so...moving. You should check it out. I know how hard it can be to find something different and unique that has just the right message.