Frank Skomial , Jul 29, 2008; 06:59 p.m.
For your short shooting time, bring up the ambient light closer to the level comparable with the bright presentation screen. This would be a momentary distraction to the 300 people, possibly waking up a few of them ?, but for the sake of taking good picture they could agree with you.
Talk to the conference organizers, or the presenter, and make a deal for a minute of brighter lighting moment.
Do you care what exactly will be presented on the presentation screen ?
Or else, if that is no possible, take two or more pictures, one exposed for the bright presentation screen, and another exposed for the dark room. Then, as you already use Photoshop, make a combined pictures from cuts out of those two properly exposed portions of the 2 pictures, and place the portions in one picture. Since you are using tripod, that seems fairly simple to do.