How about a conference? Music festival? I was at Outside Lands in GG park a few weekends ago and (given the price of a 3-day pass was $200) there were plenty of location aware smartphones used by ~60k people.
For example, at a conference like SXSW, you have x-thousand attendees with their smart phones. You can track where they all go for the whole event. But the number of people who would find that data useful (by my way of thinking at least) is quite small - the local vendors, the other attendees (who won't pay a dime for the service), who else?
How do you sell it to them and how much can you get? How do you monetize that in a way that isn't just glorified banner ads to a small market segment? How do you make it HUGE? That's where I have trouble.