Post from The New York Times news blog 4th May 2009
It would appear from the New York Times 'TheLede' Blog that the first time that the now ubiquitous @ sign appeared in writing was in a letter written written on 4th May 1536 by a Florentine merchant by the name of Francesco Lapi. Francesco used the @ as a symbol for the ancient measure of weight or volume 'the amphora'. The Spanish word for amphora is arroba and the @ sign is still known as the arroba on a Spanish keyboard.
The @ stayed relatively little used on keyboards until 1971 when Ray Tomlinson decided to use it in the first ever email address his reasoning being:
'I chose to append an at sign and the host name to the user’s (login) name. I am frequently asked why I chose the at sign, but the at sign just makes sense. The purpose of the at sign (in English) was to indicate a unit price (for example, 10 items @ $1.95). I used the at sign to indicate that the user was “at” some other host rather than being local.'