The only time I ever noticed a zipper on a character was at CP, Pooh was visiting our table and he turned around to position himself better for a picture, and there it was, inches from us. I just about panicked!!! I thought, oh NO! DD will know for sure now that they aren't "real"!! (She was 6 at the time, and showed no signs of not believing, but I knew it had to be close). I have no idea if she noticed the zipper, but her enthusiasm for characters certainly didn't wane any after that
When we first started thinking about our upcoming April 2013 trip, I initially thought that maybe we wouldn't do many character meals, thinking that DD (9 now) would be too old to be interested, and DS (2) would be too young and probably just scared. But as we started talking, it was obvious that DD still expected to do lots of them, and even started asking if we could meet certain characters that we've never seen before, which new ones might be there, etc. So it looks like we will still be spending plenty of time with Mickey and friends

Even as an adult, character meeting is still one of my favorite parts of the trip.
Just like with Santa, and the Easter bunny, and the tooth fairy... most kids reach a point where they just kind of "know", but for many it is much more fun to keep up the game and pretend. So far DD seems to be taking that approach, and I hope it continues. I was just the opposite. I held out telling my parents that I "knew" about Santa for several years, just b/c I didn't want to hurt their feelings, but by about 9 I couldn't take it anymore, it just felt stupid so i rather ungraciously told them how I felt one day while we were Christmas shopping in the mall. Looking back, I wish I would have just let myself enjoy the magic of it all instead of feeling so self-conscious about believing.
In some way, I can still convince myself that the characters at Disney are "real" even now. I've never been "friends" with a character, or known anyone that has, but I can imagine that if I were chosen to do that, once I put that costume on I would cease to be myself. In my mind, I would truly become Minnie, or Cinderella, or whoever. Their spirit and personality can be shared by anyone who knows them well enough and feels a connection to them. So if I think about it like that, they are all as real as anything else.