Whether or not the decisions have been made is not important - the relevant question is "Have all the decisions been mailed out?" The clear answer is "No," as many here on this forum have not yet heard one way or the other.
From the beginning, the USC admissions website and all corresponance has said that notification will be "By April 1st." The answer you received form the AIM representative "All students should be notified by April 1st" is completely consistant and is not new information.
Yes, of course notifications will include rejections - with over 35,000 applicants and approximately 8,000 acceptances, there will be about 27,000 rejections. There is no way around that fact. Do remember though that acceptances continue through April 1st. Check the 2009 decisions thread for confirmation.
So, to sum up, yes I agree that, as stated from the time you first checked USC admissions website last winter, you will be notified of your acceptance or rejection by April 1st.
P.S. Good luck!!!