To create these images, I attached the HV lead of my power supply to a metal plate. The ground lead was untwisted and fanned out to create a wire brush. In the darkroom, I turned on the power supply, placed a piece of photo paper atop the plate (emulsion side up), and dragged the wire brush across the surface of the paper. This did not simply burn the paper; a blue glow was visible underneath the paper, and the paper remained completely blank until I developed it.
Since I was doing this in a darkroom with other people printing, I could not risk a flash firing, so I don't have any pictures of the process. I'll pose some when I get a chance.
Obligatory safety warning: Obviously, a high-voltage power supply is a bit dangerous. If you don't feel comfortable around high-voltage, don't try this, because you'll also be doing it under a dim darkroom safelight.
Edit 5-20-08: Oh, and I got 100% and "!!" on my assignment. It's a semester-long rubric sheet, and "!!" means way cool.
I have published this under a "Attribution Non-commercial Share Alike." This means that I am not claiming to have invented the technique. It does NOT mean that you can use my images without my permission. I spent a lot of time figuring these out. If you want a high-res version (these are 600 DPI scans), I'll upload them, but they will have watermarks.