On maps.google.com you can see a brilliant example on how a google maps application can suddenly turn into a google earth application smoothly.
I\'m developing a goo
The previous answer by Fraser is perfect for V2 of the Maps API. However it's strongly recommended to use V3 since V2 has been deprecated.
To get similar functionality in V3 you can use the utility library at http://google-maps-utility-library-v3.googlecode.com/svn/trunk/googleearth/ -- simply include the javascript library, and then in your code instantiate the Earth layer for your map:
var googleEarth = new GoogleEarth(map);