Well, if we are going to make this thread complete I might as well put in a complete description.
Problem: Using IE7 (might be different in FireFox etc), when you pick any payment gateway that needs to be secured (test, authorize.net etc.) and you get a 'Page Can Not Be Displayed' error (without any other error messages it might be worth checking that your SSL is set up properly.
Test: To be sure that it is an SLL configuration issue, turn of HTTPS on the credit card page in the CBSubs: Settings -> Credit Cards -> Credit-Card Form http(s) mode: -> HTTP. And then test again. If everything works fine, it's a SSL configuration issue.
Solution: Sorry, you're on your own on this one. The number of things that could possibly be wrong with SSL and OPENSSL are way out of my league.
Mark,