Derek Chan has published a great introduction to communicating with Virtual Earth from server side code. The article is a good read to understand the basics of ajax before proceeding onto more complex examples or using asp.net ajax extensions. Read the article here:
http://www.viawindowslive.com/Articles/VirtualEarth/InvokingserversidecodeusingAJAX.aspx
Thats makes 10 articles for Virtual Earth 6, well done!