A few weeks ago XDA Developers folk managed to get Google Maps Navigation working on the T-Mobile G1. Of course, the navigation side of the software only worked in the USA and if using in Canada, Europe and anywhere else in the world you simply didn't have the option to use navigation. Yet again the XDA forums people have managed to solve this problem and Google Maps Navigation can now work on your G1 along with the navigation features out of the USA.
Details of how to get navigation working can be found over on this thread.
To get the application working with Navigation on your G1 then follow the instructions below, but I highly recommend you read over all the details on XDA Forums to make sure you get the install right. Also note that you do this sort of stuff at your own risk... ie, if you end up with a brick then don't go blaming anyone.
If you have root, uninstall the current Maps app.
adb shell rm /system/sd/app/*Maps*
adb shell rm /system/sd/app/*maps*
adb shell rm /system/sd/dalvik-cache/*Maps*
adb shell rm /system/sd/dalvik-cache/*maps*
adb shell rm /data/app/*Maps*
adb shell rm /data/app/*maps*
adb shell rm /data/dalvik-cache/*Maps*
adb shell rm /data/dalvik-cache/*maps*
adb shell rm /system/app/*Maps*
adb shell rm /system/app/*maps*
adb uninstall com.google.android.apps.maps
Next, install the SpeechSynthesis Data. You can do this either by going into the market and searching "SpeechSynthesis Data Installer", or on some ROM's, like CM 4.2.5, Menu -> Settings -> Speech synthesis -> Install voice data
If you have root, download http://drop.io/rxbqaw4/asset/maps-root-aligned-apk
If you do not have root, download http://drop.io/rxbqaw4/asset/maps-no-root-aligned-apk
Install the new file:
adb install -r maps-root-aligned-apk
adb install -r maps-no-root-aligned-apk