Agreed. Though personally, I would rebuild them both one floor higher than before and put a memorial on the new top floor of each one. Seems like a fitting tribute to the victims, as well as another "screw-you" to Osama: you knocked them down, so we'll rebuild them even higher.

There's also the fact that I'd take into account new building materials. The original design of the towers was actually quite good, and because of it they stayed up much longer than anyone could reasonably have hoped for given what they were hit with. Many lives were saved because they held out as long as they did. New materials could keep them up even longer should another attack on this scale happen, and even more lives could be saved.