Get the URL to the current xap file:
App.Current.Host.Source.AbsoluteUri
Sample:
http://www.WebsiteName.com/ClientBin/SilverlightApplicationName.xap
Get the full URL of the page hosting the xap (with QueryStrings):
HtmlPage.Document.DocumentUri.ToString()
Sample:
http://www.WebsiteName.com/SilverlightApplicationName.aspx?param=1


