I posted this on StackOverflow
If the server/site/vdir is configured using the "Require Secure Channel" setting, the response from the server will be a "403.4 Forbidden: SSL is required to view this resource." error or a "403.5 Forbidden: SSL 128 is required to view this resource.".
You can actually customize the 403.4 or 403.5 error to redirect back to HTTPS. Create a VDIR under your site with NO SSL Requirement (**This is Important) - I use "CustomError". Create an ASP File inside this directory called 403_4_Error.asp containing the following:
<%@ LANGUAGE="VBScript" %>
if Request.ServerVariables("HTTPS") <> "on" then
sServer = Request.ServerVariables("SERVER_NAME")
sScript = Request.ServerVariables("SCRIPT_NAME")
sQuery = Request.ServerVariables("QUERY_STRING")
Response.Write("https://" & sServer & sScript & "?" & sQuery)
Edit the server/site/vdir's Custom Error property for 403.4/403.5 and set the MessageType to URL and the URL to "/CustomError/403_4_Error.asp".
Note that ASP is used, you could easily use ASP.net or any other scripting language.