It looks like Apple is going to be the only store w/ walk-in stock... at least they are the only ones that have not announced they won't have walk-in stock.
BB -- Most stores are getting a handful that have been pre-sold, or not depending on various stores. Not worth trying.
ATT - said upfront no walk-in until the end of the month.
Walmart - most stores got in 4-5 which I'm sure are already sold to the manager's wives best friends. Wink, wink, nudge, nudge. Sure they are not allowed to do that. Sure.
Radio Shack - Stories vary here, but odds are they got less than BB or Walmart. I predict a lot of screaming on Thurs.
Also keep in mind a lot of people reserved iPhones AND preordered. When the preorder arrives the reservation will be canceled or not p/u. That will leave Apple stores will extra stock on Friday AM.