I agree that Apple is unlikely to allow theming - just look at Mac OS. They allowed themes (aka "skins") in OS9, but once OSX came around, it was Steve's way or get the hell out. Allowing users to customize their user interface is not likely to happen while Steve Jobs is around.
However, I disagree that people have no interest in themes. The iPhone is primarily a consumer device, and people like to personalize their belongings. What you think is tacky and ugly, other people think look awesome. And don't think this is a immature kids only club - look at the insane number of iPhone cases. Since that's really the only easily permitted "mod" people are able to do to their iPhones, the number of different designs and colors are all over the place. Really, the only people who aren't interested in personalization are probably the most devout Apple fanboys who insist on using their Apple products au naturel because that's the way Steve intended.