With the rise of twitter, it has become now fairly commonplace to use the @Username format to reply to someone. I was wondering if there would be a way to have the system detect that use to 1) create a hyperlink to the mentioned person's profile page and 2) to create a notification (à la twitter) to the mentioned person (possibly on a dedicated page instead of via email).

and

http://blog.stackoverflow.com/2010/09/new-global-inbox/

3. comments which reply using @username notation to an earlier comment you left