It's definitely not a misconception. While I do have to admit that most times it's pronounced that way tend to be in more of a joking manner, it has a basis in reality, as do most stereotypes. The first time I heard it was from my best friend in elementary school. His family had just moved to Texas from Canada and he got teased mercilessly aboot his pronunciation until he finally started saying about.
One of my bosses at work (grew up in Canada) slips every once-in-a-while and says aboot. I'll have to ask him where he's from, exactly. I've heard it several other times in the past, just don't know exactly from whom. And if you see any interviews with Michael J. Fox, you can catch him letting one out occasionally. Where's he from?