You need a good ‘central local’ to ‘hang out’ such as twitter or IRC to get a flash raid kicked off.
True. And what he said about being able to get the information in time is true...
However, I also understand flash mobs to work by ‘sharing’, ie, one person gets a text message/IM/facebook message (or other private communication) about a ‘flash mob’ and passes it on to five or 10 friends who’d be up for such a thing, and they pass it on to five more friends each, etc. One distinct advantage of this is that there’s no Twitter feed for someone to notice ex post facto that it was really pre-planned, or even notice ahead of time that it’s going to happen.
Oh, and I think showing up wearing apparel with an American flag on it (or even wearing red, white, and blue) would be effective yet subtle, and more easily deniable than carrying such a flag... you might not take any special attention of one or two people wearing such clothing, but when you’ve got a crowd, you really start to notice.
For flash-mobbing 101, there’s a book out called ‘Little Brother’ by Cory Doctorow, and it’s under the Creative Commons license if you want to download it. While it does have both applaudable and objectionable political leanings on various subjects, it’s also got a couple good chapters in which the main character organises an underground internet community and starts to get them flash-mobbing, which would serve as a pretty good how-to guide.