A little while back I made an attempt at making dust in flash, and I would basically say I failed at it (though I didn’t give myself much time). I thought I would try it again. I am fairly happy with my first test. The hardest part is that you run the risk of it looking like snow if its falling too fast, but I kept its speed up more so you can see it easier.
Each particle is looking at the background and adjusting their opacity based on the surface they are over. So if it is in a dark area they tend to fade out, etc. Note: I claim no rights to the image used in my test. It is a random grab from Flickr.
Demo (click and hold your mouse down to see the particles over black)