Okay, we don't know what'll be announced, but Facebook email could indeed dominate all email.
The deal is that a Facebook identity (profile) pretty much ensures that there's a real person behind it. It's possible to fake a Facebook identity, but it's a fair amount of work, way more expensive than getting a new gmail or hotmail account.
Let's suppose that Facebook provides an email tool, and gives you options to control what email you'll actually see.
For example, you might tell Facebook email to only accept email from people with Facebook-verified identities. You might have different levels of email priority, from friends, fans, friends of friends, and so on.
(People might link their outside email addresses with their Facebook profiles, and you might choose to accept those.)
Spammers can create their own Facebook identities to try to work around this, but that's way more expensive than getting temporary email addresses, and that raises the cost of spamming people.
So, if Facebook does this, it might provide the most personal, and spam-free email available, and it might be relatively easy to do so. That's killer.