People have been occasionally using it here on purpose since the dawn of time. Or the dawn of wotmania. Whichever came later, I guess.

It's a convention that various people have been doing for a long time, fully understanding how it's normally used here but not using it incorrectly either. He probably wasn't the first one to use it, and he's not the only one doing it today (look up some of Danae/RomaVenkat/whateverelseshehascalledherself's posts). I slip it on once in a while too. The use of "re:" has meant "regarding" or "referring to" long before the Internet came about. It's used in standard business emails and can be used to start letters. If you wanted to write about cheese you could title it "Re: cheese" and it would mean "regarding cheese".

In fact, here's what wikipedia has to say on the subject:

re has been used in English since the 18th century to mean 'in the matter of', 'referring to', or 'about'.[1] In business letters and memoranda, "Re:" may be used instead of "Subject:" to set off the topic.[2] However, "Re" in e-mail may also be used for replies.

So, y'know, na na na na-nah. :P

EDIT: And when it's used as a purposeful play on the norm, well, I will defend the premeditated circumvention of language norms, if they are used with a full understanding of what those norms are, with my last breath.