I have always had a very deep belly button. During my last pregnancy, it was totally still an innie throughout. My midwives even commented on how uncommonly deep it was, that it wasn't even "level" but still indented. DH and the midwives both commented on how bizarre it was to them how my belly button would pop out while I was pushing, it was the only time I had an outtie, and I absolutely never saw it. Pregnancy did stretch my belly button somewhat, for which I was very thankful. Before, I referred to it as being like "a pair of Chinese fingercuffs" because if you put your finger in it it was difficult to pull it back out (and it hurt!).
About halfway there this time and it's showing no signs of popping out with this pregnancy, either.
I always thought those little belly button bumps were terribly cute, too, but it seems I may be destined not to have one.