I've talked before about the notion of 3rd-party payment for a user's mobile data access, for example a venue owner or conference organiser sponsoring "free mobile broadband" for certain customers or attendees.
mBlox in particular has discussed its notion of "Sender Pays" data for chunks of content. For people without flatrate data plans, having a clear method to ensure the customer isn't going to face a big bill for downloading per-MB has to be a good thing.
However, the general contention that "Sender Pays" is analogous to the postage system's model is unfortunately rather underminded by this proposition by the UK's Royal Mail to make the "receiver" of letters pay as well....
(Note: my view is that "sender pays" content represents a small proportion of the overall opportunity for third-party sponsored connectivity, as I see content as merely a small, uni-directional sub-category of all applications. Most apps are bi-directional, with the user themselves sending a good proportion of the overall traffic. This is especially the case for user-generated content services like Qik)
Nice to know that evolution to two-sided markets isn't solely the domain of the telecoms industry.