It's an "old" story:
http://www.asile.org/citoyens/numero13/pentagone/erreurs_en.htm
which was somehow given answers:
http://paulboutin.weblogger.com/2002/03/14
http://www.snopes.com/rumors/pentagon.htm

This silly conspiracy theory obscures the real "cover-up" that seems to have taken place: People who get training on Cessnas and simulators probably can't fly a commercial ailiner with such precision into such a small building.

A couple of the terrorists had Egyptian air force training - they also trained in the US as part of joint military training. This military and political link is embarassing and furthermore suggests a complex and ambiguous story of our relationship with middle eastern "allies"; it has been left out of the popular, simple story.