Some web-based newspaper (Times Online?) held a contest* for suggestions of neologisms for this very thing. The winning entry was 'atpersand' - which I found rather feeble. The analogy is with "ampersand", the word for the symbol &.
If they really wanted to use this analogy, I feel they should have done the job properly. "Ampersand" is an abbreviation of "And,
per se ['of itself'], and". So the analogy should yield the new word 'atpersat'. But this isn't too catchy!
I prefer 'at', and if necessary I spell out 'AT-sign'.
b
PS quite recently - last year, I think