Being at 35,000 feet over Syracuse on a flight route I was flying 2x/month, and having the plane do a sudden U-Turn and steep dive as the pilot announced that we had to land at Syracuse due to a "security breach" at Logan. Turns out, it was 9/11 around 9:30am Eastern, and they were grounding the fleet. We landed in less than five minutes, then the cellphones started ringing.