Without a doubt it is harder to curve the ball now. Even the "blade" irons have taken weight (usually from the hosel) and moved it toward the toe, meaning not only a bigger sweet spot but less torqueing on off center hits, thus less curving. And the metal woods have virtually eliminated the bulge and roll effect of the persimmon woods, thus eliminating the gear effect and the pronounced curvature on off center hits. This, I believe, is the true nature of the success of the original Big Bertha. The balls, even on mishits, went dead straight. Maybe not dead straight forward, but basically whatever direction it went, it wasn't curving much.
But the big story IMO is the golf ball. Today's premium ball (of which there are many; basically all interchangeable) are much more stable in the air because of reduced spin imparted at impact, esp. with the driver. I stated in a much earlier thread that yes, golf balls are going farther (premium balls), they are also going much straighter; very little risk of curvature. And this is an overlooked reason for why everybody on Tour(s) hits it so damn far. They swing so hard without fear of it curving off the planet. Check out a junior tournament sometime. Its unbelievable how hard they are going at it! If the ball curved as much, or upshot as much, as it used to, my feeling is they wouldn't be swinging as hard. There is simply less fear today of the ball in flight getting away from the player.