It's my impression, but I can't guarantee that this is correct, is that there's a lobe of "aboot" speakers that covers Western Ontario and (we don't talk about this part) northern Minnesota and Wisconsin. There's also the possibility that it's a Scandinavian inflection, since so much of that area, on both sides of the border, is settled by Scandinavians. But it definitely exists -- my sister, who lives in northern Wisconsin, now says it, but can't hear it in herself. I once spent several weeks in Toronto, and didn't hear it that I can remember; and a couple of years ago, I traveled up to Prudhoe Bay from Minnesota, by car, and didn't hear it then, either, in Alberta, BC and the Yukon. Maybe it overlaps with Canadians who say, "Eh," as in the Great White North.
It doesn't sound like a soft "ooo" but a cross between "about" and "aboot," a sorrt of hollow "oh" sound. Peter Jennings used to say it occasionally.
The topic interests me, and if others have a different impression of its geographic range, I'd be interested to hear about it.
JC