This is really promising, though before you get excited, currently only beta for the US and Android 2.0
http://googlemobile.blogspot.com/2009/10/announcing-google-maps-navigation-for.html
This new feature comes with everything you'd expect to find in a GPS navigation system, like 3D views, turn-by-turn voice guidance and automatic rerouting.
Great stuff! And now the waiting game for NZ! Groan.
Edit: Made link work! (Posted from my mobile!)