Official comment
Edit:
As of June 20th, all new pages using any of our domains will auto-redirect to the SSL version of the page. When using the Publish dialog, check the box for "Don't force HTTPS ..." if you want to suppress the auto-redirect to SSL. Remember that PURLs do not work on HTTPS pages, so if the page is used as a thank you page with merge fields on it you'll want to not force the HTTPS protocol.
For your existing pages, if they do not auto-redirect already, open them for editing and uncheck the checkbox "Don't force HTTPS ..."