Watch the 2009 Linux Plumbers Conference (Linux.com)

however for static slides, I much prefer to be able to use the original slide rather than having to go projector to screen to camera and deal with the quality losses that imposes. turning static images into video is also extremely inefficient in terms of file sizes for the presentation.

what is really needed for interactive presentations (and I don't know if it's available at all) is a (relatively cheap) device that you can plugin to the vga port inline to the projector that can capture the video output (and ideally an audio feed at the same time so that they are auto-synced).