Class, may I have your undivided attention?

What I don't know would fill a book - millions of books, come to think of it. Today I encountered for the very first time, even though its entry in Wikipedia is nearly 5 years old,  the term "backchanneling." It was brought to my attention by this post about "How to Present While People are Twittering" ("not, you will notice, "How to "Twitter While People are Present" which is the kind of thing this old gent would worry about)  At first, as usual, I found the whole idea preposterous, but upon second thought (as usual), somewhat intriguing.  Here's one definition: Back-channelling is a way of showing a speaker that you are following what they are saying and understand, often through interjections like I see, yes, OK and uhu. [a less distracting variant is uh-huh]  In my day, that kind of thing was acceptable, within limits, for private conversation, but not for a lecture hall situation where such spontaneous exclamation would be deemed impolite and disruptive for both lecturer and listeners. Now backchanneling appears to be acceptable, even de rigeur, and here's a more modern definition:
"the practice of electronically passing notes among some or all of the audience/students during the lecture. When sanctioned, this practice is particularly useful for speakers who are attempting to dynamically modify their presentations based on immediate feedback from the audience. When unsanctioned, this practice is often very distracting for the presenter. Meebo and Twitter are common back channeling devices, although any chat room style device works well."
There's something faintly absurd about an audience broadcasting the content of a lecture or presentation while its happening, but I can't quite put my finger on it. I guess it's consistent with the need we bloggers, and  Twitterers and Facebook status updaters have to express ourselves at every moment, even when someone else is talking or nobody is listening (this blog illustrating the latter point). At least the backchannel relieves the unbearable tedium of just sitting there, listening  and thinking about what's being said. And to be charitable, the post at pistachioconsulting makes a compelling case for how backchanneling can actually enliven and stimulate audience engagement with a presentation -  but goodness how far we've come since the days of "Class, please give me your undivided attention!"