I’ve created a dynamic steam effect in AS 2, and have turned it into a class. It uses the bitmapdata class to seed squares of pixels at the bottom of the screen, and slowly scroll them upwards, while applying blur and displacement filters. It’s fast, and looks pretty realistic. One thing to watch out for is that the bottom of the generated movieclip will be very blocky, so if you can’t place it offscreen like in the demo, mask or hide the bottom twenty pixels or so.
The class still needs some more work and documentation, and I’ll be updating as time permits, but I figure I’ll put the current demo and source up so that people can feel free to tinker with it. Playing with the color parameter in particular can yield some neat, ambient effects(try #aaccffcc for an eerie green mist):