There was much buzz over the weekend about a military report suggesting the popular mass Instant Messaging tool Twitter could be used by terrorists. They cited as an example the protestors at the Republican National Convention using Twitter to inform about police actions and movement.
In that case, Twitter becomes a better counterterrorism tool than it does the next training exercise for Hamas and Al Quaida. Tweets are public and searchable which kind of kills (no pun intended) the element of surprise that terrorists crave.
In fact, Twitter has been a great source of information during earthquakes and other crisis. It's far more likely that Twitter would be a great asset to emergency response agencies and law-abiding citizens during a terrorist attack than it would be an aid to the terrorists themselves.