A speaker should definitly share their presentation at the moment they start their presentation. I'm always sitting in the back of room, at the moment the mozilla dev room, and like it was a bit crowed with the previous talk, it would be nice if you just could follow the presentation on your screen.
This reminds of a movie called "Hackers", I think, were they had some software that just could take over every screen. A bit more realistic is the idea of you logging in to a presentation server, and when the speakers continues along his presentation you see that active slide in your browser. Some dev with a bit spare time? :)
Of course you can go sit a bit more in front, but sitting in the back is some behavior of me inherited from my time at school.
Update: the mozilla dev room had just some update. It's called sauna.fosdem.org. Just says it all ...