I think this is a great discussion; just sorry I didn't get to it earlier.
I come down on John's side that golfers are just better athletes today. Of course, the equipment is better today--the most important piece of equipment being the launch monitor that can synch an athlete and his perfect equipment. But let's not put it all on the equipment. Golfers work out more today. Courses are often manicured to allow more roll. Coaching and techniques are improved. And, probably the biggest factor in increased length, is that better athletes are choosing a career in golf with the money and publicity brought on principally by Tiger.