Not only had US intelligence considered the possibility that hijacked planes might be used as weapons. The entire world's intelligence "community" knew of a concrete, imminent threat of AL-Qaeda doing just this last summer.
Last July,for example, anti-aircraft missiles were set up at Genoa international airport specifically to protect Bush from just such an attack at the Genoa summit.
See e.g. the LA Times of Sept 27 - " US and Italian officials were warned in July that Islamic terrorists might attempt to kill Bush and other leaders by crashing an airliner into the Genoa summit of industrialised natiions, officials said Wednesday. Italian officials took the reports seriously enough to prompt extraordinary precautions during the July summit, including closing the airspace over Genoa and stationing anti-aircraft guns at the city's airport."
So this was not only "thinkable" - they had a specific threat of it 2 months before Sept 11.
But when Bush gets warned about hijackings in the US, when an FBI agent is warning about flight schools, when they have a guy in custody who they think might be ready to fly a plane specifically into the WTC, everybody's mind goes blank.
But don't forget - not only did they not take any specific action, even standard procedure was forgotten on Sept 11.
When a plane is hijacked, or even suspected of being hijacked, or even starts leaving its designated course, it is standard procedure to scramble fighters. It is not a freak occurence, and it does not need any special presidential orders etc. It is a standard procedure, and the FAA and NORAD know exactly what to do when this happens.
But it took 26 minutes from when they say they knew about the first hijacking (08.20) to when they say they ordered a scramble (08.46), when the first tower was hit.
They didn't give the order to scramble to protect Washington, they say, until 09.24. The transponder for this flight had been switched off at 08.55. Two of the hijacked planes had crashed into New York and one had turned round as was heading for Washinton, and nobody ordered Washington protected until 09.24 !
And then they scrambled planes from Langley, 130 miles from Washington,instead of Andrews airforce base, which is assigned to protect Washington, being only 10 miles away.
Then there's the fact the war in Afghanistan is known to have been agreed between America and Russia by June.
Then there's John O'Neill, the head of the FBI investigations into Al-Qaeda, who quit in June, saying the investigation was being blocked by the State Department, and who took a job as head of security at the WTC (and was killed on Sept 11).
Then there's the source of the anthrax.