This article about using taxi drivers as brand ambassadors says nothing about taxi drivers disclosing that they are paid to talk about this website. I suspect this will end very badly.
Ethical nuances tend to come on the heels of, not right along with, new business models, but in the case of word of mouth advertising, it’s been pretty clearly established that you need to disclose you’re being paid. This kind of thing is cutting edge only to people who haven’t been paying attention.
Using cab drivers as information sources is old news. Every journalist in the world has used his airport cabbie as a source if he can’t find another one, and the development world has been working with cab drivers to spread health information. There’s a nice example from PSI here.