Looks like Amazon and Barnes & Noble are finally bending to Apple's new rules regarding in-app purchases. Both the Kindle and Nook apps have been quietly updated to remove the links to their respective ebookstores.
This comes shortly after Kobo, Inc. and the Wall Street Journal both announced they would no longer sell ebooks directly through their iOS apps. Looks like Apple is hitting people hard to abide by their policies. [9 to 5 Mac, WSJ]