If you install the OS X version of XScreenSaver, the phosphor saver simulates an old phosphorous terminal display. It can be configured to run any command in the terminal display, including top:
phosphor screensaver http://www.jwz.org/xscreensaver/screenshots/phosphor.jpg
I find the settings -scale 2 -delay 40 -program top work quite well, and look quite "hackerish".