Just visited google earth and it is an awesome site. I found my current house and my old place that we moved out of 12 months ago. It amazing to see even how beachmere has changed over the last 12 months. and looking at other countries also really cool, I went to italy in may last year and I looked up some of the places I went to and how awesome is it to see it all again.
For use in a a classroom, a geography assingment would be perfect, the children can look up where the assingment is based around and use that information as part of their knowledge and then reference google earth.
I have no doubt children will enjoy looking at and playing with google earth, I know I did.