Earlier this month an excellent application of Google Earth for real estate was released at coloradofuture.com (use their interface to search for houses, then select the Google Earth listings). These guys have taken their house listings and put them into Google Earth so you can look around their region of Colorado and find houses within your price range, look at the property within GE, or look for detailed information on the listing at their web site.
Google Earth is obviously a powerful potential tool for learning more about an area surrounding some real estate you are interested in. You can check for nearby things like grocery stores, malls, railroad tracks, and ATMs. You can look for hazards such as trash dumps, flood plains, and unwanted commercial buildings or traffic. And, as more cities include access to their GIS data (like the City of Portland), you will have even more powerful data to access within GE.
Significantly, the Google Earth team claims they have already had numerous people contact them that they had purchased buildings based on the input they gathered from Google Earth.
This real estate application was announced in this post at the GE Community BBS. Already, another real estate listing was mentioned in the thread as available in Florida.