Automated cabs would be a great place to advertise for nearby businesses and things. "Google, what restaurants are nearby?" which gives a list sorted by relevance ( X of Y users returned to restaurant Z more than twice ), with a few purchased restaurant advertisements proceeding them
You know, after using Google Now on my phone, I'm confident that Google can deliver context-sensitive results that make sense. Not always, not for everything, but I bet they could pull it off in a cab just fine.
EDIT: I guess it's worth pointing out, however, that Google Now doesn't show ads.
No, they are a way to more effectively collect photos for Street View and WiFi AP locations for GPS-free location based services... both are which are products of Google Maps which itself is a vessel for selling advertising.
I believe the overarching focus at Google is search. To search Google must index.
To continue indexing and searching Google has to make money. The most obvious path to billions is leveraging the indexing and searching into ad revenue.
What makes me more nervous than Google being capitalistic is Google not having boundaries about who/what/when/where/how they will index the whole world's data.
No , that is the first foray of Skynet .. much more interesting than ads.. and provably the single handed elimination of human drivers from taxi services ;-).