Other guy has a good solution to this issue before:

http://www.leastprivilege.com/PartiallySSLSecuredWebAppsWithASPNET.aspx

http://www.pluralsight.com/community/blogs/keith/archive/2009/01/17/sslhelper-get-help-running-a-partial-ssl-website-in-asp-net.aspx

Zhou Yong