Yes! It may be easy to spoof outside of Facebook, but internally FB routes it's own emails, so when you use Titan to read a message from another Titan email address, Facebook can show you thatbit's genuine,
Spammers can create shell accounts, but they can't friend you, so when you receive email from someone you don't know, Facebook can shown you their friends list and public profile, or use that to prioritize your inbox.
Soo... basically whitelisting functionality. Something that we've had for a long time. Only this is automatic since people are (hopefully) more diligent about their Facebook friends list than their Gmail/Hotmail/Outlook address book.
That's my SWAG, although you can go a little further than your friends list given a social graph. You can score an email based on all sorts of social data. Are you friends? Friends of a common friend? Do you have "likes" in common? Do you belong to common groups? Have you commented on the same thing? And so on.
Any large email player can do some of the same things with a history of emails, but FB could (I am only guessing) do interesting things.
Whitelisting is one approach to the spam problem. Inbox prioritization is an interesting problem as well. Once we have emails in your message box, you could log into FB and find 100 or more emails even after filtering for spam. So... Which emails should appear on your home page and which should wait quietly in your inbox for you to take action?
And so Facebook offers a switch: "only accept mail from friends and sites I've liked".
If you've got a popular walled garden, making the jungle outside even scarier, and thus forcing a stronger commitment to the tended 'inside', can be good business.
Although this is already the case with Google Profiles - the url is the same as your gmail account name unless you opt out/change it (most people don't)... yet google handles the spam well.