Wow, this happened sooner than we expected - but Google just announced only minutes ago that the source code for Android 4.0.1 Ice Cream Sandwich is on its way to Google's AOSP tree right now. We'll update as we learn more. Here's the original post:

Hi! We just released a bit of code we thought this group might be interested in.
Over at our Android Open-Source Project git servers, the source code
for Android version 4.0 (Ice Cream Sandwich) is now available.
Here's how to get it:Follow the instructions at out the
'ics-release' branch:repo init -u -b android-4.0.1_r1
That's it! However since this is a large push, please be aware that it
will take some time to complete. If you sync before it's done, you'll
get an incomplete copy that you won't be able to use, so please wait
for us to give the all-clear before you sync.
This is actually the source code for version 4.0.1 of Android, which
is the specific version that will ship on the Galaxy Nexus, the first
Android 4.0 device. In the source tree, you will find a device build
target named "full_maguro" that you can use to build a system image
for Galaxy Nexus. Build configurations for other devices will come
Unfortunately we still don't have our Gerrit code review servers back
online. That remains our top priority though, and we hope to have them
back soon.
This release includes the full history of the Android source code
tree, which naturally includes all the source code for the Honeycomb
releases. However, since Honeycomb was a little incomplete, we want
everyone to focus on Ice Cream Sandwich. So, we haven't created any
tags that correspond to the Honeycomb releases (even though the
changes are present in the history.)

JBQ, on behalf of the AOSP team.
Jean-Baptiste M. "JBQ" Queru
Software Engineer, Android Open-Source Project, Google.