It looks like Apple is the fastest of the Smashwords retailers, as the fantasy novella Where Magic Rules is now for sale in the iBooks store for 99¢. That means it took just over a week for the book to get to Apple and be put up for sale (counting from when it made the premium catalog, a requirement for shipping to other retailers). The novella has also "shipped" to Barnes & Noble, but I don't yet see it for sale in the Nook store.
Added Saturday: And now the novella is in the Nook store! Looks like Barnes & Noble was behind Apple only by a day.