HT to @Euan and @frogpond for this link to How to Present While People are Twittering.
There is a social control mechanism at work when you put someone on the stage, inside a room, armed with a Powerpoint deck. That mechanism locks out the world and locks out all but the most confident of the audience who might feel empowered to interrupt the presenter if they are not clear or if their information is contestible in some way.
But the reality is that there are probably several people in that room capable of doing an equally cogent presentation on the same topic with slightly different focus or with different data. Except for the regular, and frequently pointless, "question time" at the end, there is no chance for anyone to participate in the building of knowkldeg around the presentation and the format equally empowers the speaker over the audience who get to throw balls that the speaker hits out of the room or lets slide, there is no actual dialogue.
Twitter unpicks those locks and enables the co-construction of the information stream, it dissolves the walls, probably makes the PPT worthless because the Twitterers are not looking at it, they are hunting down and sharing the original data, or some contest of it and helping each other through the process of understanding, which is what the presentation ought to be about in the first place.
I'd love to see whether open Space events such as Johnnie Moore's produce more or less back channel activity. I'd guess less because the interactivity is up front. I also suspect it would matter less in Open Space than it appears to be in conventional forums.