Bernhard Sterzbach has created an overlay to view Google Maps in Google Earth. Here is an excerpt from his website:
Here's a network link for Google Earth which will use Google Maps to give you a transparent map overlay for the area you are viewing in Google Earth.
When I was entering placemarks for my recent holiday in Japan I noticed that Google Earth resolution (outside Tokyo) is fairly limited but Google Maps shows all of Japan with excellent detail. So here's my attempt at integrating those two services by putting a transparent Google Map over the Google Earth view. Generating the links to Google Maps was fairly straightforward, but getting the alignment right was quite a challenge.
His site contains all the information that you will need to implement this.