A) That won't happen again, security has been tightened
B) He could never have made it to the President
So it is without precedent. At no point in American history has the President of the United States been beheaded by a terrorist. In our current system, there is no way a terrorist could get to the President to do so.
So let's stop freaking out over the rantings of improbable actions by crazies. The President is in no more danger than he was previously, the terrorists cannot get close enough to behead him.