This year (2008) my team and I have been measuring our training successes with and without the use of Powerpoint visuals.
We measured several key statistical areas: Content, Instructor, Environment, and an Overall category.
When foregoing the Powerpoint visual, our Content scores are up 18%, Instructor scores are up just over 20%, Environment scores are up 9%, and the Overall category scores are up 17%.
In each case, the instructor(s) remained the same for both versions of training. No matter the instructor, each time we measured results, the scores were higher in all categories! We attribute the improvements due to an increased connection between the instructor and the trainees.
If you have conducted similar research, please let me know. Until then, get back in touch with your students -- Abandon the Powerpoint!