piatkow - 10:10 am on Oct 13, 2010 (gmt 0)

In model #2, where the user deposits money in advance, do banking regulations enter into it if the operation, say, becomes large enough?

Depends on the jurisdiction. My understanding of the Transport for London scheme is that it isn't covered as long as they restrict usage to paying TfL fares. (Quickly adding the qualification that I am not a lawyer and left the banking industry over 20 years ago).

I would add that anybody planning to take deposits from customers in this way should be careful to check all local and national regulations in their jurisdiction. (For example you may be required to hold the money in a dedicated client account)