Instructions for current wheezy versions:

  # apt-get install plymouth plymouth-themes-solar
  # plymouth-set-default-theme solar

Then rebuild ISO (including re-generating initramfs), boot with "splash" bootoption and "/etc/init.d/plymouth" needs 
to be stopped as part of the boot process, then it works pretty much out of the box.

Nobody cared about the implementation, so marking as dead.