this is what I did:
Step 1:
first download the source code from
git://android.git.kernel.org/platform/frameworks/base.git

step 2:
 copy the \base\core\java\android\ to android-sdk-windows\platforms\android-8\sources
 you need create the sources folder yourself.

Step 3:
 restart the Eclipse.

for 1.5 source code,
please see this article:
http://geekycoder.wordpress.com/2009/07/03/how-to-download-latest-zip-android-source-code-and-using-it-in-intellij/