From what I understand the policy is roughly: If you are selling something that can be used "outside" your Android app, then you are not required to use Google'

From what I understand the policy is roughly: If you are selling something that can be used "outside" your Android app, then you are not required to use Google's payment processor and you can avoid the 30% cut. Amazon (and other ebook vendors) use this to sell books directly from their reader apps. I'm not sure what Dropbox, SkyDrive, etc do.