I think it was Sports Illustrated did an article a couple years ago about driving distance on the PGA tour. At the time, if I remember correctly, the average drive was about 280. What SI did was break down the tournaments into 3 categories: hard and fast, damp and wet, and soaking wet. In the hard and fast category, the average drives were 280. In the damp and wet conditions, average drive fell to 260. In the soaking wet conditions, the average drive fell to 240. Of course, there's always someone that throws everything off, John Daly averaged over 300 in all conditions.
Net result? Yes, equipment and balls have added distance, but IMHO, the number 1 enhancement for PGA players is course condition.