"To border on becoming unacceptable by established risk conventions -- that is, to reach an annual fatality risk of 1 in 100,000 -- the number of fatalities from terrorist attacks in the United States and Canada would have to increase 35-fold; in Great Britain (excluding Northern Ireland), more than 50-fold; and in Australia, more than 70-fold. For the United States, this would mean experiencing attacks on the scale of 9/11 at least once a year, or 18 Oklahoma City bombings every year."
I'd say it was time to get a grip, but to be honest, that time has long come and gone.
"risks are deemed unacceptable if the annual fatality risk is higher than 1 in 10,000 or perhaps higher than 1 in 100,000 and acceptable if the figure is lower than 1 in 1 million or 1 in 2 million. Between these two ranges is an area in which risk might be considered "tolerable.""
- Schneier on Security: Terrorist Attacks and Comparable Risks, Part 1 (view on Google Sidewiki)