Eric Dolecki pointed me toward a new editor on OS X, weirdly named Smultron. Regardless of its name though, it is a pretty good editor, which has support for multiple documents within the same panel / window. It has all of the basics you would expect from an editor, and also appears to have built in support for ActionScript.
It is written in Cocoa and is open source, so if I can ever get it to compile, maybe I will add some more ActionScript support.
If you are on OS X and doing Flash development, you should definately check this out.
You can find more info, as well as download the editor from here.