If you're not familiar with it, it's difficult to call it a "game" because it's actually just a tech demo; there is no objective, no scoring, and no way to lose a life. That said, it's remarkably fun to drive around various places in the world with just a few presses of your keypad. As long as you have the Google Earth Plug-in installed (which you most likely do), you can play the game here.
If you enjoy driving around in the milktruck, you'll really enjoy Paul van Dinther's Drive the A-Team van" game from a few years ago. It's the same basic idea, but with 3D trees, collision detection on the buildings, and missions to accomplish.
This is one of the great things about Google Earth; old games/files will automatically improve over time as better imagery and new 3D buildings are released!