Anarchogeek has a wonderful collection of tweets that illustrate the somewhat disorganized history of today's commonplace @reply function. The first use of the @ symbol came on March 25, 2006 (just four days after Twitter was created) as a location check-in, and was also used to designate time or just to replace the word at. The symbol wasn't used in conjunction with a username until November 23 of that year, at which point the Twitter community struggled to establish a standard of use. For more on what led up to Twitter's addition of official support for the @reply feature on May 12, 2008, check out the full story at the link below.
@kapowaz: probably— Neil Crosby (@NeilCrosby) November 23, 2006