Yesterday on Twitter, there was some shatter about Apple rejecting applications that include the Dropbox SDK. MacStories has an excellent article on the issue which is summarized here:
Apps that integrate with Dropbox, in fact, can either authenticate through the installed Dropbox app, or, if not installed, open a web view to let users log in with the browser. The alleged problem with Apple is that the Dropbox mobile login page contains a link to go back to Dropbox’s main website/account creation page, and possibly purchase a subscription bypassing Apple’s App Store (and thus 70/30 revenue split).
At the same time of this controversy, release 3.2.411 was approved by Apple! My application (Ultimate Password Manager) is using release 1.2.1 of the Dropbox SDK in order to allow users to backup the encrypted password database to Dropbox. This is not a central feature of the application.
My take is that if your application's main feature is depending on Dropbox in order to work, Apple may reject your application. Release 1.2.2 should address this issue according to Dropbox Developer center.