iPodHacks has a very easy way to display your iTunes visualizer across multiple monitors using Mac OS X's built-in accessibility zoom feature.
The instructions show you how to enable Zoom in the Universal Access system preference pane. When the visualizer is playing, you can zoom in on the iTunes window, and the image of the window is spread across all your screens.
The quality of the zoom, of course, is a bicubic interpolation of the screen image, so it's not as crisp as it could be. Also, the fluidity of the display depends greatly on how powerful your video card is, and how it's connected to your computer. Multiple video cards also don't necessarily help, unlike with the original Magnetosphere visualizer.
For a party, though, it's a neat trick. If you have other ways to make this happen, feel free to leave a comment!