Why is it, when you install an app from the AppStore that requires an account, and you don't already have an account on the service, you have to fill out a form and create one?
It seems like a dumb question, until you consider that this is not your computer.
On your phone, you're always already authenticated with your Apple or Google account. How long will it be until the iOS and Android SDKs offer developers one-touch authentication services? It could be a great experience for users, and a great service for developers. Not to mention, highly strategically valuable for Google and Apple (not so much for Facebook). Especially when they add the other pieces of the identity puzzle.
Update: I was right! :) I shoulda done more homework: Since I haven't used Android extensively, I missed it, but I'm told this does exist on that environment. I need to see it in action to know if it's what I'm talking about. But I will update my prediction to say that, after Apple follows suit, this becomes the primary way people register for new services (over the long term). Eventually, sign-up forms will become obsolete.