http://www.whatreallyhappened.com/WTCDEMO/wtc.html
Check out the part on the WTC building having been dropped by US authorities on command. I'm fairly sure this would take lots of work, coordination of involved personnel, local police/safety personnel, and weather (no/weak winds). Suggesting that the WTC towers were dropped intentionally would mean that explosives would have had to have been placed at the proper places (requires engineering blueprints), and detonated without anyone else knowing. Furthermore, dropping the towers with people (especially police and fire personnel) in them would be a serious command decision. Who would have that authority..? Certainly not any police or fire officer. I don't think Guiliani would have authorized dropping any buildings (assuming there was time enough to properly prepare for a precision drop) with NYPD and FDNY personnel in them. The only other people who could authorize that would have been the Governor of New York, or President Bush. Seeing as Bush was being "secured" elsewhere, I doubt he would have given that order.
Getting back to the logistics of a precision drop, the decision to do so would have to have been made almost right after the planes hit the towers. Then the appropriate personnel would have had to have been brought in (remember all air traffic was quickly shut down), they would have had to obtain enough explosives for each tower (in the confusion of that day), obtain and analyze the building plans, emplace said explosives (while people were streaming in and out of the WTC), and then detonate (with said people inside). All the Discovery Channel shows about this subject I've seen has the crew working for days (in abandoned buildings) and carefully re-checking their work before detonating.