Юзаем на хостинге:

HttpWebRequest request = (HttpWebRequest) WebRequest.Create(http://www.ya.ru/);

выдает:
Security Exception Description: The application attempted to perform an operation not allowed by the security policy. To grant this application the required permission please contact your system administrator or change the application's trust level in the configuration file.
Exception Details: System.Security.SecurityException: Request for the permission of type System.Net.WebPermission, System, Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 failed.

долго думали :-)
читали Google Group
добавили в web.config:

<system.web>
<trust originurl="" level="Full">
</system.web>

заработало!!!