I am quite interested in this... but...
a) I don't think I want the Thread tagging (separate tagging, not @). Can it easily be disabled?
b) I think using the semicolon is going to be too much for dumb users

What about the option of making it something more like BBcode, like [@username] and that way there is also a toolbar icon (like a megaphone -> shout icon), and the dialog input box can just ask "username to shout at" or whatever. Just an idea.
c) I see on your forum you're already considering group/category shouts, that sounds like a very good idea.
d) Kind of a follow-up to (b) now that I think about it

My forum usernames have this regex "^[A-Z0-9 ]+$", with that in mind I am wondering if there is a simpler way as to not require the semicolon to activate the tagging...
Mike