A small but notable development at Facebook means that if you're replying to a comment posted on your profile, you can now do so through regular e-mail.
A post on the Facebook blog explains it: "These notifications--for comments on such content as status updates, photos, videos and Wall posts--allow you to stay informed about your Facebook friends' activities without being logged in when you're on the go, on your phone or at work. Today, we're launching the ability for you to participate in these conversations by replying directly to these e-mail notifications."
So, basically, if someone comments on something you've posted to Facebook, you can respond and add an additional comment by replying to the e-mail notification that alerted you to it in the first place.
For more, read "Finally! Comment on Facebook through e-mail" from CNET News.