See
http://en.wikipedia.org/wiki/Sign_of_the_horns
Lennon (sorry Mika) was apparently up to in '67. Of course he'd been reading... how did you guess... Crowley at the time.
Then, and this really surprised me at any event, it became a big deal in Parliament/Funkadelic/Bootsy Collins circles. For those of you think there's some weird stuff around NOW, apparently you had to make the sign of the horns to gain admission into the mothership. Well, George Clinton always did have an active imagination.
Maybe Heavy Metal is more sensible...