The feature, , doesn't appear available to all users yet - myself included. My hunch is it will take hours, maybe days, for the functionality to be available to all 350 million Facebook accounts.
E-mail Replying to Facebook Comments 101
Now when you receive an e-mail notification from Facebook alerting you that someone has written on your wall, you can respond with a comment of your own simply by hitting reply, typing your response, and hitting send. The body of your e-mail appears on Facebook next to the original comment. No longer must you click on a link in the Facebook e-mail, go to Facebook.com, sign in, and then post a reply.
In order for this feature to work, make sure that your Facebook notification settings are turned on (do this by going to "Account Settings," then "Notifications," and checking the appropriate boxes). The feature will only work when replying to e-mail alerts regarding status updates and comments left on photos, videos, and wall posts. Unfortunately, you'll still have to log onto Facebook to reply to private messages.
This latest Facebook addition may be small, but it is a tremendously useful one for people who like to do everything from their e-mail account.