Wife got an email from her sister yesterday morning to the same effect about girlfriend, 9/11 and malls on halloween.
As in all things these days, I'm skeptical. I'd probably be more inclined to chalk it up as an urban legend at this point. The threat of terrorism is very real, but I don't think we're going to see it telegraphed like this.
Of course, it's also possible that the story is a disinformation campaign of the terrorists to get us whipped up further into a state of paranoia, without actually having to do anything.
Halloween is always a good excuse for rumours and I've heard way too many over the years. Personally I think we have enuf to dread with all the drunk lunatics getting bent out of shape in their Bacchant celebrations. Halloween seems to be the biggest excuse for acting like idiots that we enjoy as a society.