Eclipse foundation recently have released Eclipse E4 Milestone 1 with the support of so called ActionScript development environment for SWT. It means that you can now export Java applications for the Flash Player. There is some pretty good instructions to run some demos. I've played with it last night. This post is intended to give you a quick overview of the whole thing with an ActionScript developer point of view.
The installation process is easy. You mainly have to download Eclipse SDK 3.5M4, install some updates, the Adobe Flex SDK 3 and configure the Eclipse environment. It takes less than an hour if you have a good connection and sufficient knowledge of what we are talking about. Just do not forget anything and cross your finger, as the E4/SWT team said: "[..] both the tools and the port are in the pre-alpha stage. You are living on the bleeding edge. Good luck!".