As of Android 4.0 the integration of the Android SDK source code is much easier.
You can just download the source code via the Android SDK Manager. The sources are downloaded to the source directory located in “path_to_android_sdk/sources/android-xx”. xx is the api level number (15 for 4.0.3). You can then add the source code as a source attachment to your android.jar.
I included this info in my Android Tutorial.
Thanks to Ed Burnette for pointing that out on Google+.
Kudos also the the ADT team which made this great improvement in development experience.