With the recent introduction of full html and javascript capability for Facebook Fan Page iFrame tabs, it only makes sense that Facebook would add HTTPS capabilities. Some users browse Facebook only in HTTPS browser mode, which means they are browsing Facebook securely.
Well when someone visits your iFrame tabs in secure mode, if your apps are setup with an HTTPS URL, then the tab won’t be presented.
This will also allow app developers to potentially integrate payment methods and credit card forms to their Facebook Fan Pages. With an HTTPS canvas URL, the page where the credit card form resides would be fully secured.
To make your iFrame tab HTTPS capable, simply add your URL to the “Secure URL” field in your Facebook app.