If you haven't seen it yet, check out the article on moving to an open source Flash development environment written by Carlos Rovira.
Towards Open Source Flash Development
The article does a good job in explaining how to use MTASC and Eclipse to develop .swf files. Being that I'm now helping to develop ASDT, I thought I'd link to the article.
Also, on a related note, congratulations to Nicolas Cannasse (MTASC author) for being accepted as a speaker at OSCON 2005. Flash at an open-source convention is an awesome step forward. Best of luck! I hope this will draw more developers to the Flash platform.
All we need now is an open source component framework...

Killer, will check it out.
Thanks for the info!
-erik