This has been dealt with time and time again. 1stly, I said nothing was done by Bush et al in response to the terror warnings. This is stated by the 911 Comm. Please don't ask me to prove that they are not lying.
2ndly, you do not appear to know what a PDB is. Every weekday morning, the president and some principals meet with the DCI (Director of Central Intelligence), who triangulates the most important intel for them to hear. ~30 weeks b4 911, and 40 warnings. I.e. he was told more than once every 4 days that AQ were planning an attack on US interests; zero follow up. These warnings were "unprecedented" in their scale, according to Tenet.
Well, it's certainly clear that YOU don't know what a PDB is, mjd1982.
It's not a briefing. The "B" doesn't even stand for "briefing." It's a "brief" -- a document compiled by the intelligence community that is hand-carried to either the President or his Chief of Staff every day. The DCI neither briefs nor even meets with the Pres every day, and never has. It'd blow three hours of his day for no useful purpose.
As far as specific, imminent warnings before 9-11, I have yet to hear of one that was credible and specific enough to be actionable -- though if Janet Reno's directive about not passing info to the intel community from the FBI had never occurred, it might have at least delayed 9-11. Maybe. Jihadist fools are in no shortage, and they can be trained to fly anywhere in the world if you have money.
Every day the intel community gets tens of thousands of pieces of intelligence, and a lot of them are warnings of one type or another. Most of the ones that aren't pure crap are so vague you can't do squat with them except wait for better data. Even if the CIA had reasonable intel that AQ was going to try suicide missions with planes, what's the next step? Ground civil air? Tighten up security, which with the usual airline non-cooperation would take years? Once this happened, even the usual moronic jihadists would modify their plans to use rented planes, incoming Swedish airliners, or several other obvious alternatives.
The unfortunate fact is that it was, and still is, easy to pull off a mass murder in an open society, especially if suicide is an element. It is now more difficult to use regular commercial flights as kamikazes, but any idiot could still pull off a similar spectacular media event. And they will.
As before 9-11, anyone with half a brain knows that we are going to be hit again. Big time. Might be planes, and we have no real defense against a decently-planned hit by incoming foreign flights, and zero time to mount any defense if they coordinate all of them decently. It might be a couple of Scuds from a perfectly legitimate cargo ship offshore -- or, if we're unlucky, a couple of Silkworms, or a crude thermonuclear weapon. If we're lucky the nuke will be sent to some city and do some ferocious local damage. If we're not lucky it'll be configured for an air burst aimed at maximum EMP damage and reduce a significant portion of the Eastern seaboard to pre-industrial conditions very rapidly.
This isn't just good intel -- it's a simple prediction obvious to everyone paying attention. That's the easy part. Now go figure out what to do about it.