Yep we got bit by this one. It brought our network to its knees. Fortunately the fix was easy. First, to stop the attack we turned off ntp temporarily:
service ntp stop
Then we added this to our /etc/ntp.conf file:
Disable monitor
Restrict -4 default kod notrap nomodify nopeer noquery
Then we started ntp:
service ntp start
The system returned to normal.