You can now use Google Latitude on your iPhone, though it is just a webapp. Interesting information form Google on why they didn't build a "traditional" app:
We worked closely with Apple to bring Latitude to the iPhone in a way Apple thought would be best for iPhone users. After we developed a Latitude application for the iPhone, Apple requested we release Latitude as a web application in order to avoid confusion with Maps on the iPhone, which uses Google to serve maps tiles.
Google, like Apple, continues to push for improvements in web browser functionality. Now that iPhone 3.0 allows Safari to access location, building the L
