Think about it. There no doubt were all kinds of criminal activities planned on telephones in the 20th century - should that have led to their banning or any across-the-board restrictions? Aside from being an unconstitutional violation of the First Amendment, such limitations would have been entirely unnecessary: we already have restrictions on criminal activities.
Banning or even general limitations on flashmobs not would not only violate the freedom of speech provision of our First Amendment, but its freedom to peaceably assemble, as well. And they are similarly unneeded. We already have ample laws on the books against looting and other criminal activities of crowds. England does, too.
The Arab Spring, while not successful everywhere, has already peacefully spread to democracies in Israel and Spain. The larger message of these assemblages of people, brought together through online invitations, and publicized through Twitter and other new new media (my name for media which transform consumers into producers) is that we may be witnessing a profound shift, even in democracies, from representative to direct forms of governance. When elected representatives don't do their jobs, the people press to take more power. This was always the case - and why we in the United States changed from selected to directly elected U.S. Senators a century ago. But now the voice and wishes of the people can be heard as never before, through the smart phones in an increasing number of hands.
Governments would be wise to take this revolution seriously, and not disable it by even a well-meaning but unnecessary limit on smart phones and flashmobs in response to a summer of hooligans.