I've always thought the math on this was sketchy. Where are there enough users concentrated to make checkins commercially useful? I just can't see it, but I reserve the right to be totally, completely, wrong.
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.