I was at a tech conference yesterday that had its presenters up on a stage with the screen for presentations directly behind them. Big problem? The seated presenters heads were obscuring the view of the bottom 25% of the screen. If you are a conference organizer, you should look at a room as it will actually be used. This means you should sit in the audience and have someone sit in the cahir where the panelist is and see if anythign is obscured. This will solve the problem.