A few default responses:
Try some boot cheat codes:Boot Methods for special ways of using Sabayon
Try increasing the verbosity and noisyness of udev by loading your LiveCD/DVD/USB session, mounting the Sabayon installation, and editing etc/conf.d/udev, and turn on debugging by removing the "#" from the line: #udev_debug="YES"
Then reboot to see if you can find out where udev is hanging.
You can also do a Google search for "Sabayon waiting for uevents to be processed", but these problems seem to change over time, and the old answers may or may not be relevant.