Wirelessly posted (iPhone: Mozilla/5.0 (iPhone; U; CPU iPhone OS 2_0 like Mac OS X; en-us) AppleWebKit/525.18.1 (KHTML, like Gecko) Version/3.1.1 Mobile/5A347 Safari/525.20)
Go to settings and you will see an AIM setting option there. Select it and turn on "sign off on exit" and that should do the trick.