GPLify is a Perl script that adds notice of the GPL to a collection of source files. It can alternatively use a customized template file to put at the top of source files. It requires String::Trigram.
What I should really do (and would if I wasn’t at work and in the air) is modify this program and release it as BSDify, which would of course be released under the GPL license. How much brain-hurting fun would that be?