Like Kallison & Buckalew, we went to WDW less than a month after 9/11. Getting on that plane truly created so much anxiety for me, but once I was in WDW, I was so glad we went.
Ya gotta live your life, and not sit and wait for something horrible to happen.
I can't say in the future what would make me cancel a trip, all I know is from my past experience, and that was to go as planned.