I have started to see what to me is a new convention in multi-response blog comments, the use of @ plus a previous commenter’s name to indicate that the current comment is aimed at that person rather than being a response to the original post: for example (to quote one example from Language Log today)

Ryan Rosso said,
July 8, 2008 @ 10:06 am

@jagorev: I would agree

Have I been hit by the recency illusion in thinking this has only just started happening? Does anyone known when it began, and why the @ symbol was chosen, rather than (as would seem more logical to me) saying to: or something similar? (Comments on whether this is an unnecessary and irritating affectation are probably ultra vires ...

I know this has been a convention at Metafilter for quite some time. In the best cases, the responder links the “@jagorev” directly to the comment they are responding to. It’s also a convention on Twitter. However, I have no historical data on when or where this started, nor is it easy to search for.

I know this has been a convention at Metafilter for quite some time.

Among a few people; I’d say on the whole it’s deprecated.  The usual MeFi convention is to put what you’re responding to in ital and let the reader figure out who said it if they want (as I’m doing here).  It is much more common elsewhere (BoingBoing?).