According to this listing from Walmart's website, the Transformers: Revenge of the Fallen novelization by Alan Dean Foster is already in stock on their site and ready to be shipped, despite earlier information that the book would not hit shelves until May 29th.

Discuss the news in this Allspark thread by lonegamer8. Credit to Seibertron.

UPDATE: It appears that the comic-book adaptation of Transformers: Revenge of the Fallen (written by Kris Oprisko and illustrated by Alex Milne) is also available on Walmart's site here.