WebRequest and SSL (The underlying connection was closed. Could not establish trust relationship with remote server.)
Wholly Handy, Batman.
I hit this error and was stuck. My application was sending Web Service calls from one machine to another across SSL and I had only tested straight-HTTP. Once the errors started coming I realized I need an SSL testing environment. Nonetheless, an implementation of ICertificatePolicy was the answer. No wonder bloggers are Time's people of the year!
Note: The reason for my error is that we were using the IP address and the common name on the SSL certificate was not an IP address so there was not a match. This was the same error IE gave me when I would nav to it in the browser. This is such a common problem with SSL - it's not unwise to ignore the common name IMHO.
I might add that this is not a great approach in certain circumstances when you need to handle the expiry of certificates and other things. In the case of that, there is a sample on MSDN that shows what to do Here.
Update! Having the problem still bug me and the implementation not work as expected, I did some more searching and found this MSDN article that shows how to make a change in the Machine.Config to make this a global solution without a code change.
In the Machine.Config change