Karen Corby, PM working on the Silverlight 2 HTTP stack, posts an awesome overview. Her first post covers the site-of-origin policy (also known as the cross-domain restriction) and the two APIs used for HTTP communication: WebClient and HttpWebRequest/HttpWebResponse.

http://scorbs.com/2008/04/05/silverlight-http-networking-stack-part-1-site-of-origin-communication/

Yavor Georgiev
Program Manager
Connected Framework