New features on Google Earth have just been unveiled and show the increasing detail of information that Google is using to really map the earth (and sky). The main new features:
You can see any earth scene in the lighting from any time of day
Street View, the actual photos that Google is taking of many cities, is integrated into Google Earth
Users can help Google build the "street view" of Google Earth - take a picture of your house (or other building), use Google Sketchup to turn it into a 3D image, send Google the 3D image and address and they will decide if they want to add it to Google Earth.
Here's a great little video from the Google Earth blogger illustrating some of these features; he also has more information and screenshots on the blog.