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Good question. You'll, I hope, be happy to know that Flash CS4 and Flex 3 both run on ActionScript 3.0. There is no time line from Adobe on when or how ActionScript may be updated to a subsequent version.
But when that happens, you can bet that we'll be working with Rich and Zevan to update *Learning ActionScript 3.0*.
If you'd like to learn more about how Flash CS4 works with ActionScript 3.0, though, I'd recommend you take a look at a book we released in October, to coincide with the CS4 launch: *ActionScript 3.0 Quick Reference: For Developers and Designers Using Flash CS4 Professional*.
Hoping this helps,
Executive Editor, O'Reilly Media