As far as tools go, I am aware of Haxe, MTASC, and swfmill. Can you offer any success or horror stories related to any of them? Are there any others that I should be investigating?
With respect to learning, the Adobe Developer Connection seems to contain decent reference materials, but the tutorials all assume that the reader is using the "Adobe Flash" product. Are there any tutorials out there targeted at Open Source users?
What are the advantages of Flash 9/ActionScript 3 versus Flash 8/ActionScript 2? Am I correct in thinking that Flash 8 is still more widely deployed than Flash 9, and better supported by the Open Source flash players?
If you are going to be targeting mobile devices (or the Wii) you will probably want to stick to Flash 7.
Adobe still doesn't have a Flash 9 SDK available to those (embedded/mobile) platforms. The newest SDK is FlashLite 3 which supports Flash 8.
Anything build before or around Oct. '07 would most likely still be using the Flash 7 SDK though.
Of course, if you are only developing for Mac/Win/Linux then none of that really matters.
I've always been of the opinion that if you don't need any of the new features or bug fixes it's generally best to stick with the slightly older but more compatible versions. That applies to pretty much any development.