During a terrorist attack — during any crisis situation, actually — the one thing people can do is exchange information. It helps people, calms people, and actually reduces the thing the terrorists are trying to achieve: terror. Yes, there are specific movie-plot scenarios where certain public pronouncements might help the terrorists, but those are rare. I would much rather err on the side of more information, more openness, and more communication.
Schneier on Security: Communications During Terrorist Attacks are Not Bad.