I'm not sure it's as simple as that. Most Dropbox users are using the free service, so really the sign-up is no different to the sign up on a hundred other apps that can link to an online service. In fact, I would argue that the vast majority of people linking to Dropbox from within apps already have an account, and if they wanted to upgrade the account they wouldn't be doing it from a link in some PDF viewer app they downloaded.
What worries me is that Apple might be using this as a lever to decrease the relevance of Dropbox because it breaks their app-centric model. I love my MacBook Pro, iPhone and iPad, but it's Dropbox that makes them usable for real work. Steve Jobs said that MobileMe was a disaster because it didn't work - I would argue that iCloud is even worse - it's taken away the iDrive and replaced it with a total mess. My jaw hit the floor when I realised that the method for getting files back onto my Mac after editing them on the iPad is to log into the iCloud website and download them one by one onto my Mac. Total disaster. I've been defending Apple on Slashdot for 3 years now, but if this is the start of a war on Dropbox then I would seriously consider dropping the platform.
"Don't hate me because I'm beautiful. Hate me because I'm beautiful, smart and rich." -- Calvin Keegan