There is one step, though, that Obama can take to help heal some of wounds inflicted by religious bigotry and rebuild respect for the Rule of Law. I'd prefer that he take the step of not even having an "invocation," but that's not going to happen... and would be needlessly rude at this stage after extending the invitation.
Nothing in the law says that the oath of office need be taken with a hand on the bible. Although I think Bartlet was right in rejecting the Sports Illustrated Swimsuit Issue as an acceptable substitute, there's another weighty tome that would be more than acceptable: The annotated Constitution. What more could one do to show both Americans and others that the single officer most charged with enforcing American law felt that the source of that American law is "sacred" enough to form the foundation for an oath? (It might also go a long way toward undermining the "My deity/religious dogma told me to break the law" excuse so common throughout the world.)
My ultimate point is that anyone who is worthy of being the highest legal figure should not need assistance from some imaginary being to uphold that duty.