Latitude isn't enabled in the UK, and some other countries, on the T-Mobile G1, however it is enabled on many other phones e.g. Nokia N95. This is frustrating, a Google phone without Google Latitude.
Latitude Launcher can enable Latitude for you if you have 'Rooted' your phone. It checks for a row in the Gservices table, if it doesn't exist it adds it and launches the Maps application.