I mean, it's not like attendees are talking into a phone, inconveniencing other people. Complaining that every attendee's attention is not permanently fixed upon you strikes me as particularly vain. What about taking notes on a laptop? What about scribbling into a notebook? You should be glad that someone's interested enough to repeat the things you're saying to others.
Backchannel tweeting to me sounds like an excellent solution to the whispered conversation. Now you don't need to be physically proximate to someone in order to discuss ideas raised during the presentation. Not only that, but you're not going to disturb other attendees doing it! Thanks to this tech, people can add richness to the presentation they are attending.
This leads into the future. What happens when we have implants that allow us to communicate without tapping on a device at all, but rather just by thinking? How are you going to even know who's fully fixated upon you, and not engaged in backchannel conversation? And would you still be offended?
Always remember: Presentations are not for you; they are for your audience.