Like Uncle Albert, I can't tell you specifically about all the precautions WDW has in place to protect the safety of its guests and cast members but I can tell you that a LOT of what WDW has in place, our Gov't has learned from and followed suit in some way.
They literally have cameras covering every square foot of the property. from the bathrooms to behind the trees.
A short comparative story of WDW vs. Universal:
I visited both at XMAS 2002. I went to Universal 1st. They have rent-a-cops that briefly check your bags. I was seated on a bench in Universal where I witnessed a man showing his 8 yo son, his taser. They were shooting it at the birds. I was horrified! Moreso, in the fact that this man managed to somehow conceal this and bring it into the park. Very calmly I walked over to a Universal employee and asked for security. They arrived about 10 minutes later and I explained what I saw to them and pointed out the man. They said they could do nothing. I said if they were unwilling to detain him then I would contact my friends and they would detain him. I also said that I would call the press and Universal would get bad publicity. They got rather irate and asked who did I think I was. I replied as I pulled out my own badge, "A federal agent." They did go over to the man but I didn't wait around.
I went back to my Universal hotel and checked out. I drove to WDW and stayed there. I have told WDW about this, as well as my co-workers - tasers are illegal in public in FLA and, now, in most states as well. WDW is VERY Safe and VERY good at it.
As for 9/11 - we were all caught off guard. We can all take steps to ensure that it doesn't happen again by keeping our eyes open and being alert to situations. WDW is doing just that.
SO go, have a great time, and don't worry!