I guess I'm in the minority here, but I would find this incredibly annoying. With all due respect, I would not use this feature if it were created. And I think it would be fairly difficult to implement to be honest. The @ sign can be used for many different purposes, not the least of which is email addresses. Using it to trigger a username lookup is going to add overhead to the posting process, especially since if I am reading this correctly you anticipate this happening live, as I type the post? Granted the username field should be a unique index so the query process should be fairly efficient.
But ultimately what are you trying to accomplish with this feature? I can't think of a single time that I wanted to link to someone's profile. I want to link to a post, a topic, or some other specific resource on the board, but never to a user profile. And what happens if a username changes? The link no longer works. If you covertly store the user_id in the post and replace it with the username for display purposes, you can address that. If you're parsing the post for a username, I would think you would have to do it each and every time the post is viewed since it can't be static. It's not like BBCode, where once I format something as bold it stays bold. Something that is a user name today might not be tomorrow. What if user accounts are banned? deleted? or as mentioned already, renamed?
And frankly I don't care to know if someone is talking about me.
Just my two cents. I would rather see energy put to use implementing some of the other ideas than this one.