I think LH has the right idea, much as I hate to admit it. This is Mr. Obama's oath, and must be in the form that he sees as most binding upon himself. As a believing Christian, any oath that did not mention God would not be as meaningful to him as one which did. Whether or not someone else "affirms" or professes a lack of belief in God/god entirely or finds the inclusion of a/the deity in the oath of office repugnant isn't really relevant.