Do you work with the SDK nightly builds or the Flex SDK svn repository? If yes, then you have probably faced the situation where the flex public language reference is slightly older (compared to the nightly builds). If you are working with some new APIs then you obviously want to look at the latest asdoc.
Here is what you can do:
1. Go to the root folder of your local SDK (doesn’t matter if you are using a SDK zip or the public svn repository)
2. Run the command:
ant asdoc
This command will generate the asdoc for all projects that are part of the Flex SDK, and the documentation (in html format) will be created in a folder named “asdoc-output” inside the sdk root folder. It takes around 7-8 minutes, so have patience (it is going through a lot of source files in the SDK 
)
It is also a good option if you sometimes work offline or can’t access internet for some reason.
Couple of caveats:
1. It only works from the SDK root folder
2. Only available in builds for FLEX 4.
3. You will get the documentation only for projects that are in the FLEX SDK, so don’t freak out when you don’t see the documentation for Flash Player classes or charting classes etc.
You can grab the latest nightly builds from http://opensource.adobe.com/wiki/display/flexsdk/Download+Flex+4
Hey, nice post, really well written. You should post more about this.