This Wired Article sums up Tufte’s analysis…
“Slideware may help speakers outline their talks, but convenience for the speaker can be punishing to both content and audience.”
His Cognitive Style of PowerPoint essay costs $7 and is essential reading for anyone who wishes to avoid the mind numbing effects of this invasive tool. An excerpt, PowerPoint Does Rocket Science, is available online. Also visit Ask ET and his weblog.
Update: Visit my Using PowerPoint Effectively resource page.