Im very impressed with ActionScript 3.0 and how all the events work. I have been reading up lately on progress events specifically the open, progress, and complete events. You can manipulate these events in so many ways, it makes creating a preloader a breeze. For example, you can use the progress event to create a listener that tracks the progress of a file that is being loaded into your flash file. You can accomplish this in about 3-5 lines, then you can create an Event.COMPLETE listener that checks to see when the file that you are loading is complete then it calls a function which does something. Its so much easier than having to create 10-15 lines of AS2.0 code to determine how much has been loaded, etc.
I will be working on a script/demo to explain this further, look for it in the coming weeks.