In reply to aln:
theres an interview with iggy pop that (like every interview he does) asks about just this.
hes an articulate guy, and in the interview he says the teerm 'punk' goes back to old 40s gangster movies like the Maltese Falcon, meaning some upstart with more verve than ability. then apparently, 'punk' got applied to young rockers in the 50s as a description of their attitude, not as a label - 'a punk rocker' meant a rocker who had more enthusiasm and bravado than talent (apparently there were non-punk rockers who had the reverse).
by the time the stooges, MC5 etc appeared in the late 60s they were being called 'punk' to describe their attitude and aggression without refined talents.
no doubt like 'hippy', 'freak' and 'rapper' it was then taken up by the music journo crowd to lable something they felt needed packaging, which lydon/mclaren et al then both milked and took the piss out of ever since, with wankers like U2 and matchbox25 claiming a piece of it.
by the time the media got onto it im sure the real players had been throwing the term about for ages.