Someone wrote a HOWTO for building Android 4.0
http://www.cnx-software.com/2011/11/24/android-4-0-on-beagleboard-and-beagleboard-xm/
And pre-built images are also available:
http://sola-dolphin-1.net/data/android/ICS/ics.beagleboard.tar.bz2
http://sola-dolphin-1.net/data/android/ICS/ics.beagleboard_xm.tar.bz2
Enjoy!
Christian