According to a blog from Capcom, Street Fighter IV has sold out in Japan.

They have confirmed that they do have more stock readily available and will ship soon, but their first shipment (which is normally called "one day allotment) has entirely sold out.  They heard from producer Yoshinori Ono-san that Capcom does have a few copies left in the basement which they are going to release into the market.

As to how many were sold has not been revealed yet, and even Capcom says that numbers that have been published are way off base. Guess we will have to wait and see later this week, as to what the actual numbers really are.