I.E. If you make an entry "friends only" to read, only people on your friends lists can read it. But lets say a friend makes a comment to you and you want to put a private answer on it that only they can read.
As far as I know, this isn't possible right now.
I think it would be handy, however.
Although I suppose the counter-argument could be made that email could accomplish the same idea. But if you wanted say 3 people to be able to read the comments, instead of all your friends, then it seems feasible for that purpose for the private comments to be useful.