Seth Godin points us to this article which says PowerPoint is bad for your brain.
To be fair, the problem is not PowerPoint itself, but how people use it.
This quote says it best:
"It is effective to speak to a diagram, because it presents information in a different form. But it is not effective to speak the same words that are written, because it is putting too much load on the mind and decreases your ability to understand what is being presented."