Cameron's comments about Twitter really are beyond stupidity (and I note the commitment was to look at social networking - leading me to suggest that when the time comes nothing will be done).
Leaving aside the fact that banning the medium of communication is a ludicrous response to a situation (nothing made Thatcher look so foolish as the "oxygen of publicity" ban on Sinn Fein, even if it did provide employment for a number of Gerry Adams sound-alikes):
i) It would probably need primary legislation to do it - it's not at all obvious that the Government has the powers
ii) It is quite likely to be contrary to the European Convention on Human Rights, which protects the right to correspondence (of which, lawyers would argue, Twitter is no more than an electronic form)
iii) Quite a lot of police forces use Twitter to get messages out to the public - including messages about where disturbances were taking place in the last few days (and more significantly where disturbances were not
taking place, in order to quash rumours - for example, followers to the Sussex Police twitter feed were informed that rumours of disturbances in Brighton's London Road on Tuesday were not true. Since I was standing at a bus stop in the London Road at the time I knew this already, but it was reassuring)
iv) Twitter is a business whose revenue model is based on levels of activity (it gets a cut when people post to it using their phones). Is the Government going to compensate Twitter when it is switched off?
It's a classic example of a political establishment more interested in the Daily Mail line than the truth. It's just one example IMO of Cameron's poor performance in the last week - a weak man using big words to try to look strong, without bothering to find out whether those words make any sense. It's also I think an example of something I've been thinking for some time - that the Coalition just isn't very good at Government