A month or so ago, I posted a review up about an app called City Story. Since then, there have been a few complaints from readers who have incurred excessive charges. The screenshot above (which clearly warns of an In App Purchase), was taken during my testing of the game, and appears in the review. I inquired to the vendor what could have contributed to these cases. A spokesperson for the app responded (see below), but be sure to always pay attention to prompts on your phone when playing this and similar games, and is not advisable to let small children play unattended.
"I think that it's the same as how your iPhone pings you about anything else. For instance, if you d/l something and input your password, then for the next 5 or 10 minutes or so it doesn't prompt you again. I'm pretty sure this is what people are referring to, but I think it's pretty clear that the user is making a purchase. It alerts you each time you're buying cash."
Jim Karpen has also pointed out an article in TidBITS that indicates some ways to help prevent this by tweaking your settings.