In his Nov 30, 2011 post, Jean-Baptiste Queru explains why the kernel source is invisible after cloning. This post describes the kernel source branches that exist for the various hardware flavors of ICS.
And in general, here's how to make a specific branch visible:
git clone https://android.googlesource.com/kernel/common.git # clone the repo
git branch -a # lists branches, both local and remote
git checkout -b android-3.0 remotes/origin/android-3.0 # create local branch
# android-3.0 that tracks the named remote branch
