Since installing Flash CS4 a few weeks back, I have been searching my hard drive looking for the offline help and ActionScript 3.0 language reference. Using the offline docs can be much faster than waiting for the online version to load, and they are helpful on planes or in other places you don't have internet access. A great resource to bookmark, which is what I did in CS3 but haven't been able to do in CS4 since they were not in the same or a similar location.
Well, it took me quite a while, but I finally found the correct location in CS4 and thought it might help save someone a few minutes of searching by posting it here:
C:\Program Files\Common Files\Adobe\Help\en_US\AS3LCR\Flash_10.0\index.html
You can also find the entire offline documentation for the Flash CS4 IDE at the following location in OS X:
Note: Since I only have a copy of Flash CS4 for OS X, I don't have the path for Windows. If you find the docs location on Windows, please leave a comment and I'll update the post.
[Update: Windows path added. Thanks!]