Well, you'd think so, wouldn't you?
Most terrorists, as well as almost all soldiers, are opportunists.
It's innate to the concept of guerilla warfare that you can't strike where the line is strongest, so you wait for the enemy to drop their guard. This typically means they plan a means of attack without a specific target, and wait for their target to present a window of opportunity.
The attacks on New York on September the 11 2001 were the most lethal terrorist attacks ever. It seems quite likely that, in view of their success, those responcible or with similar motives will try to make use of such tactics in the future and this board could potentially be a major source of information to them.
I agree that there it's quite likely that that post alone presents little threat, but there is no point in putting ANY potentially damaging information that doesn't need to be public knowledge into the public domain. The old addage of "loose lips sink ships" rings very true right now.
Consider this: If someone had came to you on September 10th 2001 and told you what was going to happen the next day, would you have believed them? Or would you have considered it to "far fetched" to actually happen?
It's easy to be wise AFTER the event, but right now this isn't a good option.