I personally prefer Flash Builder for writing the code. Flash is a pretty terrible coding environment. I find it much easier to lay out my classes and interfaces in Flash Builder.
I'm not sure I've divided the packages up the best way I can. I may have to reorganize. I put the letter 'i' in front of the interface names to make them stand out from the classes once I have added those.
For those who aren't familiar with Interfaces or what they are good for, most object oriented languages only let you extend a single class (C++ being a notable exception). You can, however, implement as many interfaces as you like. Since on screen objects will need to either extend MovieClip, Sprite, or some other DisplayObject I'm going to use interfaces as the primary way the game engine will interact with the display objects. Functions within the game engine will take the interfaces as parameters, and deal with the objects indirectly that way. Hopefully this will make the engine more extensible so that anyone who wants to use it for their own side scrolling games will be able to do so with less difficulty.
Hopefully I'll have more to show soon.