Finally the dark closed source chapter in the history of Android is closed, as the Android 4.0 source code has been made available.
With the release of Android 3.0 Google decided withhold the source code of Android, and promised that Android source code would be available in a future release. That future release would not be in the 3.x series.
Because all the Android source code is stored in a Git repository, it is possible to look back in the history of the code to get the Android 3.0 “Honeycomb” code. According to the Google’s own Android Open-Source Project engineer Jean-Baptiste M. “JBQ” Queru, Honeycomb was a “little incomplete” and developers should focus on “Ice Cream Sandwich”. There aren’t any tags demarcating any particular state of the Android source code repository as Android 3.0.
However with the history intact, this does mean that the Android 3.x code is open source as well, and can probably be used for Android hacks such as CyanogenMod.
You can find instruction for downloading and building the Android source code here, and can read the original announcement about the availability of the source code on the official Google Grouphere. Beware though, compiling the Android source code is not for everyone, even if the procedure is straightforward; the source code itself is a 6GB download, needs upwards of 25GB for building (over 80GB for a full set of builds), and needs 16GB of RAM.