The new core 2 duo's use less power than their P4 older siblings. For 4 years I had a Pentium 4 2.8ghz (no HT) Northwood Core and the heatsink/fan spun at a higher rpm than the stock one that came with my new Core 2 Duo E6400. It also runs about 5-10C coolor depending on load on the CPU.
Now thats not to say other components are getting 'cooler.' Newer hard drives have more platters and are stacked closer together, which generates more heat. Northbridge chipsets on motherboards also run hotter these days...and the list goes on.
