Comment-discussions are becoming very inconsistent if users change their names, so it might be a good idea if @username would be automatically replaced by @userno on submission and autoexpanded to the current username on loading the question. Since this lookup is also done when the replied-to user's comment is loaded it wouldn't require additional loading anyway.
edit (in reply to @Mark Hurd 's answer) ok, I thought the userid is stable. But as long as there is a way to internally connect comments to its poster, the same mechanism could be used for the @-expansion
update Since questions now have a permalink with userid-referalpermalink with userid-referal, I assume the userid is now stable, so this could now be implemented...
