They definitely intend to deliver ads
So instead of just easily setting up an automated email system to inform users they are close to their cap, they spend time and money testing this system? Just to deliver a message about the status of an account? Yeah right.
They have every intention of using this for delivering ads later on. They are introducing this tech gradually though to get people used to the idea. They are trying fly beneath the radar.
They cant possibly hope to justify this as the best method for informing a customer. What about if I dont open my browser for a few days whilst im reading binary newsgroups or downloading via bittorrent? How do I get warned then?
This has nothing to do with helping customers stay informed, and everything to do with plastering the screen with ads at some point later. No doubt about it.