Interesting suggestion, but ~ I blow out my PMs on a regular basis, and the gallery of "added me to the list" in my case, includes a couple of peeps I've never done the PM with.
Call me crazy (I know, too easy), but other than our Mods? Nuke this feature. Nobody, and I mean nobody, should be able to "follow" somebody else's posts all over the forum. It just reeks of creep.
That said, kat, I'd like to think it's just some bizarro anomaly that happened on the side with Neil and co. doing some major reconstruction here. But I've been there, been stalked. I believe it's a weirdo feature that we just didn't understand is there unless we really worked at trying to follow somebody around in this particular fora set.
Did I mention call me crazy, because it's just so easy?