Actually, Dean Cain only started wearing glasses recently, so that theory wouldn't apply.
There was never any explanation given in the show, so fanfic explanations are all we have. There have been a number of stories which address this, but most authors seem to go with the route that he began wearing glasses at some point during his childhood as a reminder not to use his heat/xray vision (or to prevent him from accidentally doing so). Some people have also suggested that wearing them also made him look more like his parents, which may have been important to him as a child, especially once he started questioning his orgins. He may also have started wearing them as something of a disguise, even before becoming Superman. For example, we know that he often helped people secretly before coming to Metropolis. This is why he had to move around so much, because people would get suspicious.
As for it being a "slip up", however, I don't buy that ... to me, that's like saying someone is "out of character" in the pilot episode. How can someone be out of character when the early episodes define the character? The show's creators wanted Clark to have started wearing glasses before his arrival, and so he did so. It can't be a "slip up", whether they explained it on the show or not. It's just a given.
Kathy
