Taken from the Google Blog:
“A few weeks ago we launched Google Maps Navigation (Beta) as a free feature of Google Maps on Android 2.0 devices. Today we’re expanding availability of Google Maps Navigation to devices running Android 1.6 (Donut) and higher, such as the T-Mobile myTouch 3G and the G1.
This release also includes the new Layers feature, which lets you overlay geographical information on the map. View My Maps, transit lines, Wikipedia articles about places, and more.
If you have a phone running Android 1.6, you can download an updated version of Google Maps from Android Market to use Navigation today. Google Maps Navigation is in beta and is currently available in the United States. Some features of Android 2.0 are not available on Android 1.6, for example, the ability to use the “navigate to” voice command.”