"Yet most Catholics I know believe in Limbo. And I would bet they have no clue is not official dogma."
I was always disturbed by the idea and never personally believed in it.
But yes...many catholics do believe in it because it was in their catechism (a mistake on the part of those who printed up the catechism) and drilled into their heads at parochial school - or during sunday school.
For me personally it never jibed with the idea of a merciful God. I used to think "He found a way for sinful adults who are guilty of terrible sins, and He is helpless to save the innocent babes?"