I think that people's perception of CGI does change over time. When My dad and I went to see The Two Towers, he came out of that movie thinking that Gollum was a real guy with makeup. I could have been fooled as well, except that I knew he was a digital effect going in. Now I look at Gollum and see the signs of a digital creation, but that's because I've seen so much of it that I have greater awareness for that sort of thing.

It's not that people aren't good at recognizing trickery, it's that we often don't know where to look to find it, at least upon first viewing. To make another Two Towers example, I had watched that movie probably a dozen times before I realized that in Theoden's final cavalry charge down the causeway, the horsemen in the back of the procession are flailing their swords at imaginary orcs. Now I cannot unsee that, if only because I now know where to look to see the seams showing.