Android Kernel Download
The main Google repository with Android source code is at: http://android.git.kernel.org/
There are (as of September 2009) 4 main separate kernel repositories at that site:
To download one of these and use it directly, you can use git.
To add one of these to your overall Android repository scheme, you add the appropriate kernel repository to your local_manifest.xml file. (???)
Note that when you download the rest of the Android source code, using the 'repo' command, you do NOT automatically get a kernel tree included. (That is, a kernel git tree is not referenced in the default Android manifest file, when you use repo following the instructions at http://source.android.com/download
For more about repo, see http://source.android.com/download/using-repo