Well, I can agree and disagree with that. Moving from alu to carbon made a huge difference in ride for me, but one could argue that the ride on the alu bike was dictated by the oversized tubes. But I also have a Scattante, which does not have ridiculously oversized tubes, and it still rides way harsher than my carbon bike (same wheels and tires).

My wife has a Lemond Reno (alu), and it has 32h, 3-cross wheels with 700x25 tires. When she test-rode a steel Serotta, the first thing she noticed was that it was MUCH less harsh of a ride, even though it had a radial front wheel and 700x23 tires.

The material may not be 100% responsible for the ride quality, but I think the choice of material often dictates how the bike will be built, and the material + design together are what determine the ride characteristics.