I too share your interest on the details of the actual limit of practically unlimited plans. As you probably know, what was virtually unlimited for data for most smartphone users just a few years ago wouldn't be satisfactory for today's emerging multi-media, continually streaming, HD+ usage levels. I feel your frustation for technical vagueness or inaccuracy in the use of the term unlimited. However, similar to various "4G" technologies that are 3G+ and other varients for speed notations in the cellphone industry, from a competitive marketing and easy to understand consumer point of view, some ambiguity in definitions is useful in simplified communications.
Thank you for the practical example to illustrate in concrete terms the likelihood of there being an appartantly undisclosed upper limit. I would like to add there is the consideration the variety of types that make up the user pool. The overall usage of the unlimited usage proram prediction has to take into account low, average, power, and ludicrous data quantity users of their service. This makes it difficult to set and publish actual standing hard limits both in the initial launch of the feature and upon scaling usage levels. How would you convey this in a brief ad or commercial to satisfy your need for accuracy without being overly complicated and long-winded in consideration of how the competition will respond and in their own marketing messeges?