Well, bedwetting pullups are mentioned in one episode of Arthur revolving around one character worried about her bedwetting when invited to a sleepover, but I don't think she ever actually wears it.
There is an episode of The Proud Family where Oscar has to stand on a street corner wearing a diaper because he lost a bet.
Kim Possible has an episode where Ron gets passed back and forth through and age regression machine one too many times, so he's still wearing a diaper when he returns to his normal age for the last time.
There's a cutaway gag in Family Guy about Peter wearing adult diapers. He pauses mid-converstation to use it.
In Fairly OddpParents, aside from getting regressed to a baby, Timmy is in a diaper at one point at his normal age.
There's that scene in Lost where Charlie and the rest of his band wear diapers for some kind of advertisement.
There are others but I can't think of them at the moment.