There is a lot of noise going around about Apple’s comparison of the MacBook Pro vs. the PowerBook G4, and the iMac Core Duo vs. the iMac G5. According to the SPEC benchmarks used, the new iMac gets 3.2X (integer) and 2.1X (floating point) the performance if the iMac G5. The MacBook Pro gets 4.5X and 5.2X of the performance on the same test over the PowerBook G4.
People seem to have overlooked that Apple actually posted some real world benchmarks on their Universal Applications page.
According to their graph, the MacBook Pro is 1.7 to 4.1 times faster in the 6 applications they tested, and the iMac Core Duo is 1.5 to 2.6 times faster. Still, these benchmarks cover a limited scope, and they don’t tell you exactly what or how things were tested in most cases… But check it out anyway, it’s better than nothing at this point.