That's my thought, too. One of the problems - aside from the press' love of sensation - is that those in charge are caught in a bind. If they announce it, there's panic in the streets and, of course, they look ridiculous when nothing happens. However, there is just that outside chance that increased public vigilance will prevent something. The flip side is that they don't announce it, something happens, and we bring down our own government with the partisan fighting that breaks out afterwards.
This would all be a lot easier if it weren't taking place in the midst of a political game of "gotcha." I think Bush, by not focusing on Clinton's many security failures until very recently, was trying to avoid this very thing. But the Dems have been playing the game since a few months after 9/11, long enough for the trauma to wear off for them, apparently, letting them go back to their old ways.
The real threat comes when white-hot hate of the terrorists combines with that of the Dems.
Like it hasn't already? The only difference is that terrorists send bombs, and Democrats send bums. The targets are the same.