I'm not sure how you can even ban Twitter at a conference? How would that work? Pretty odd.
I'm all for setting twitter etiquettes during conferences. That way, sane people can still express their thoughts in a manner that helps. And dicks can be dicks for the world to see.
The twitter stream should be projected over the speaker, like supertitles at an Opera. See Steven Colbert's "Word" sketches or Mystery Science Theater 3000 for inspiration.
I'm all for setting twitter etiquettes during conferences. That way, sane people can still express their thoughts in a manner that helps. And dicks can be dicks for the world to see.