One of the pluses (well, or minuses) of working at a fast-paced interactive ad agency is that a lot of times, you will have to learn as you go. We recently worked on a Flex 2 project that required us to learn AS3 on the fly. By no means the learning curve was as steep as jumping from AS1 to AS2, but getting into AS3 wasn't exactly a walk-in-the-park either. I am sure there will be more and more resources on learning AS3 as we get closer to the release of Flash 9 and Apollo, but here are some of the resources we found useful during that process:

• Flash 9 Preview on Adobe Labs
• Adobe AS3 Language Reference
• AS2 to AS3 Migration Cheatsheet
• AS3 Tip of the Day
• AS3 Tutorial from Flashscript.biz
• Actionscript 3.0 Cookbook by Joel Lott

Also you might want to pick up Essential ActionScript 3.0 by Colin Moock when it comes out in June