http://gawker.com/5744229/the-facebo...ly-as-possible
Facebook today is rolling out an option for a user to be signed into their entire session via a HTTPS connection. It is an option that you have to enable under Account->Account Settings->Account Security. I would highly recommend everyone enable this feature, especially if you travel frequently and are on open/public wifi networks. The setting should be rolled out to all users by the end of the day.
It sounds like this will be the default setting in a few weeks.