A few months ago, Facebook announced a completely new messaging system that they planned on rolling out this year. The new system included offering @Facebook.com email addresses to users.
They are now issuing a few beta @Facebook.com email addresses by invitation only. You can request to be put on the invitation list by going to the Facebook Messages page and clicking on the green button at the bottom.
How many beta addresses are being issued is still unclear. Also unknown is when Facebook intends to make the addresses universally available.