It is not an app, but instead a decent web-based version.
Google has finally released a version of its Latitude geo-location service for the iPhone. However, Apple forced encouraged Google to abandon its native iPhone app, citing possible confusion with the default Maps app. Even though Google was able to develop a nice looking web-app alternative, the result leaves much to be desired. The basic gist of Latitude is similar in some respects to BrightKite—fire up the app and it sends your GPS coordinates to Google. It also lets you view a map showing the coordinates of all your friends, wherever they might be located. If you’re trying to meet up with friends on a Friday night, for instance, it makes finding them much easier. Like BrightKite, users can control who can see the location information and just how accurate it is—helpful for avoiding awkward confrontations with that stalker-y blind date from last week. – From Ars Technica