I think this is a timely article since we where just discussing this.
http://www.linux.com/article.pl?sid=06/09/29/164207
Executive Summary:
A buch of GPL code ended up getting distributed in binary only. Over a bunch of smally ports the license files ended up getting dropped.
Key point of the article for LinuxBIOS:
""" As Doelle points out, some of the problems might have been avoided if the GPL header had been included in each file, rather than depending on a single COPYING file containing the text of the GPL. At least this would make it more obvious to developers what the requirements are for their use of GPLed code. """
So we should probably do as Uwe says and start including the GPL header on all source files.