Well it isn't exactly 100% actionscript, I cheated a bit. The effect was made by using a number (100 I tihnk) of "dot" movie clips moving in random directions and adding a Flash 8 blur filter to them. This makes them almost invisible on a black background, but when 2 or more overlap you get a more visible spot. When many overlap, you get this cool looking effect. I've used actionscript to move the objects around the scene. You'll be surprised how easy it is to make something like this once you know the basics.

If anyone wishes to obtain the *.fla source file for this effect please let me know.