In our projects so far, we used the free Notepad++ to edit our .mxml files. It does nice syntax coloring (if you add .mxml as an extension for XML in the style configurator).
But now, in the mailing list the question came up whether you can also use Adobe’s professional editing software FlashBuilder. In fact, Adobe is giving Flash Builder away for free to the education sector.
The answer is yes, you can use FlashBuilder to create scriptingRT experiments.
After download and install, simply create a new project, then save a copy of scriptingrtXXX.swc in the libs folder, and you are ready to edit your .mxml file. To test, use Run, to export the finished script, Export a Release Build from the Project menu. This also builds a html page.
I haven’t tested this much, but I can see two immediate advantages: The syntax coloring works for both XML and ActionScript in the same file, while in Notepad++ the embedded ActionScript is not colored. I imagine that you also get enhanced debugging options this way.