Well, AAdvantage Terms and Conditions say no: "Transactions prior to your enrollment date are not eligible for mileage credit." (http://www.aa.com/aa/i18nForward.do?...Conditions.jsp)

That being said, I suspect if you called and asked, there is a decent chance they'd go ahead and give you credit anyway... especially given how they advertise it in-flight (the whole "If you're not earning advatnage miles for this flight, log on to AA.com to sign-up..." line sure makes it sound like you can get credit for the flight you are on)