This question stemmed from a discussion here: Ad networks that will serve via https?
My question to the OP was "is it necessary to have every page behind SSL?" Admittedly at the time of posting this question, the OP hadn't got back to me to let me know what kind of site he had, was everything behind a login etc etc
I agree that if everything was behind a login then yes, it should also be protected by an SSL certificate, but if (for example) the site was a wordpress blog, would it be necessary for the parts of the site that were just going to be viewed by your average user (i.e. one that's not logged in) to be accessed over HTTPS. Admittedly, if you were logged in and viewing the "front-end" of the site, it would send over your cookie, but as long as you took precautions against that then it should be ok, right?