We use this as a droplet, so we can convert dropped FLA files to PNG in a shot. The original script is much larger, as PNG files are post-processed before using them (need some tweaks in GraphicConverter or a similar apple-scriptable app), but it is a skeleton you can use in many little automation projects involving the wonderful world of AppleScript and Flash Javascript (aka JSFL), the language used to automate and extend the Flash IDE capabilities.
The FLA file will be exported as PNG using the last settings you used to export a PNG file.
You can download it here.