'Til now it is just philosophy, and I doubt it will ever become more than that. As far as I see it, Brian Greene is wasting his time on something that - like fastlane mentioned - we won't be able to test in our lifetime.
Thing about Superstring Theory is also that so far it isn't able to make any predictions that could give the slidest proof of string theory being correct. Don't forget, we didn't test general relativity through experiments. It just made some predictions about the world surrounding us that happen to be considerable accurate. Therefore we assume that Einstein was right - he doesn't have to be, since we know that either general relativity or quantum mechanics have to be at least a little bit wrong - though we can't actually see the warps and ripples suspected by general relativity.