Depends on the type of terrorist.
If it is a random person who hates MJ, then there will be no major impact on history. Sure days of mourning and a possible second memorial for both MJ and the fans/celebrities killed in the attack, but nothing more.
If it is from a group in the US or a person saying that they are promoting this groups purposes ((IE: KKK, Westboro Baptist Church)), then there would possibly be a nation wide crackdown on domestic terrorism. Now Obama might be fair and go after all potential groups (both Left (eco-terrorist) and Right (Christian Extremist) ) or (as some people think) he would be biased and only go after the Right Wing. (I find the latter unlikely).
If it is from Al-Queda, then Obama has two choices. He might decide to increase US presence in The Middle East. If this happens then The Right will point out that McCain/Bush was right, and it is Obama's fault that an attack occurred; and The Left will be furious that Obama, after promising to pull out of Iraq, has decided to send more troops to The Middle East. If Obama decides to not increase presence, then the American People will (most likely) be outraged at the fact that even though we were attacked that we didn't do anything about it, and this could possibly make America appear "weak" in the eyes of enemies (IE: Iran, North Korea, Al-Queda). Either way, Obama is screwed.
However no matter who carries out the attack, a few things are sure to happen. MJ fans will be furious and heart broken. This attack would also cause MJ's death and life to be remembered forever. In time the fact that he molested young boys could be forgiven by society. So in an Ironic way, an attack on MJ will only increase sympathy for him, and thus in time support.
