I've heard the prediction before that Harry will lose his magical powers (by way of jheaton), but I don't really think that's going to happen. There's no precedent of a witch or wizard losing magical abilities, as far as we've seen in the earlier books--though that doesn't rule it out, since canonically there was no precedent of anybody surviving the killing curse, until Harry did as a baby.

It just doesn't make much sense, story-wise, for Harry to lose his magical abilities. He'd be cut off from the whole wizarding world, which is the only real home he's ever known. And what kind of ending is that?