A Continued Commitment to Security is the Facebook blog post on how they are enabling HTTPS security on their system.
Starting today we'll provide you with the ability to experience Facebook entirely over HTTPS. You should consider enabling this option if you frequently use Facebook from public Internet access points found at coffee shops, airports, libraries or schools. The option will exist as part of our advanced security features, which you can find in the "Account Security" section of the Account Settings page.
If you're concerned about security, things like Firesheep, and not allowing people to get into your facebook account fairly trivially, then this is a good thing. Head to the settings page on your facebook account and enable this now.