We have released the complete docs for the Flash MX 2004 extensibility layer. This allows you to script the authoring environment and create news tools, commands and extensions. I personally think this is the coolest and most useful feature of Flash, and I have already used it to create a command line compiler for the authoring environment.
There are a couple of extensions on DRK 5 which use the extensibility layer, and you can also see a list of third party extensions here.
You can view the docs on-line here.
Have you had a chance to play with the extensibility yet? If so, post your thoughts and / or links in the comments section.