OK, the title is a bit of a put-on. I have noticed, since moving to Philadelphia from New England, that some people insert an extraneous “all” when forming certain questions (e.g., “What all methods of reducing dandruff have you tried?”, or “Oh, You went to the barbeque, Who all was there?”). Not to single out an individual poster, but I have noticed it in Wesley Clark’s threads
and I notice he is from Indiana (BTW, sorry, Wes. Unfortunately there is no way to use the SDMB search engine with the phrase “what all”, as the search terms are either too common or too short, you are my example because it is prominent in a number of threads you have started. Absolutely no offense intended).
I had never heard this colloquialism until arriving in the Mid-Atlantic region. In some cases, it appears that the asker means for respondents to specifically enumerate all possible results/answers, but I cannot say for sure that this is always implied/consistent with its usage.
This has me wondering… What all feedback can the teeming hordes provide?
I grew up in rural Louisiana and although I am pretty sure I have heard some people used that construction, I am pretty sure it isn’t that common in Southern English.
I should the above comment was for “What all”. “Who all” is common in the South.
Do we say “what all”? I don’t think I hear that. I do hear “Who all was there?” And of course “Which of youse is…?”
I hear them both fairly often, here in the midlands of SC. I do hear “who all” more than “what all”, but I hear them both.
I was born and raised in Southern California and reside there now. I use and hear “who all” moderately, but certainly not exclusively. “What all”, however, just sounds a little funny.
Mother used both; she was from Tennessee.
I’ve heard “who all” frequently in Arkansas, and “what all” to some extent.
It all seems perfectly natural to me, though I can’t be sure what all phrases I use in everyday speech. 
Born ‘n’ bred near Chicago.
Interesting - If I heard that expression locally (Cape Town, SA), I’d have a reasonable suspicion that the person was first-language Afrikaans or spoke English that was more Afrikaans-influenced than the norm, like the Cape Flats Coloured dialect.
I wonder if there’s an appreciable/historical Dutch influence on the American version?
Just a thought.
I hear both of these in Arkansas. Not only that, if a group of visitors is encountered, we might ask, “Where all are y’all from?” If a person is going on vacation, we might ask, “Where all are you going?” (In both cases, it is implied that more than one location expected in response.)
I thought this was standard in Southern speech. It’s really the same thing–pluralizing a pronoun (who, what, where, or you) that has no distinct plural in the prestige American dialect. (This was pointed out to me by a professor in college.)
I had a friend who used both phrases. He’s Canadian, but he probably got it from his father, who was a transplanted Dane. It’s about the only instance of such local usage that I can recollect though.
“What all” is perfectly normal around here. Maybe it’s a midwest thing.
I thought it was, like, valley speak, dude!