According to the publishing info, Last of the Jedi -- the first of the new young adult series that focuses on Obi-Wan's wacky adventures following Revenge of the Sith -- should be released in May.
However, I saw quite a few copies at the Waldenbooks in Temecula, CA, on Saturday. (They also had the first issue of the ROTS comic adaptation out a few days early.)
Furthermore, both amazon.com and barnesandnoble.com have the book listed as "in-stock," while the other ROTS-related books have a "preorder" status.
In any case, the actual physical books appear to have been sent to stores, so I'm willing to bet money that they'll be released on Saturday with the rest of the ROTS books. (Hey, if Wal-Mart's sitting on a ton of books that they know can't be put on the shelves because they're SW books, do you think they're going to notice a later shelf date on one book out of about twenty-something?)