Big Spaceship released a nice AIR app today which allows you to download and view the entire AS3 language reference locally. However, you don't need an AIR app to view the language reference offline -- there are actually two other options.
I had previously posted about the offline help and language reference locations in Flash CS4 which allows you to access the local copy of the language reference from within a web browser. But did you know that you could also access these local files from within the Flash CS4 help panel?
Normally when you access the help panel in Flash CS4 it automatically tries to load the online version of the documentation and language reference. I assume Adobe chose to do this so they could keep it updated and you could benefit from other user's comments in the Live Docs.
To access the local copy of Flash CS4 help and the language reference, just tell Flash to “keep you offline”:
Go to Window > Extensions > Connections. In the panel that pops up, click the arrow at the top-right and select “Offline options”. From there you can check the box for “Keep me offline”. Flash will then always default to using the offline help.
I should note that doing this may also prevent you from using any other panels which call out to the internet, such as Kuler and the Flash startup/new file screen. As of now I'm not using any Internet-connected panels and I have the splash screen set to not show on startup, so this works quite well for me.