Note: Facebook has closed this service due to less popularity.
Today when i logged in to my Facebook account, I found a small pop-in message near profile image asking for sign-up for email i accepted the request and in some click I got my email id. But it is looks to be a IM instead of full email. you can send email to firstname.lastname@example.org and receive also. You can also attach some files.
Beside this Facebook email is not a full featured email. Since it has still a small compose window and feels like we are still using older features like commenting, chatting.
I also get 10 invitations with the sign-up and used all at the same time. 10 invitation is quite less because average Facebook account has 300 friends. Another thing I will add that this sign ups are only for those who have pretty profile name instead of profile number.
e.g. If you have facebook.com/myname than your email address will be email@example.com and you also not be able to change your facebook email address in future after opting once. I think this is going to worst since your email will be public and causes a lot of spam.
I have attached some screenshot for you..