He used iTunes to benchmark these systems. He converted songs using the "Convert Selection to AAC..." feature and noted the rip speeds as displayed in iTunes.
Machine Model - iTunes Rip SpeedAccording to these results the new MacBook Pro is quite a bit faster than the PowerBook G4 but slower than a PowerMac G5 system. You can check out the forum thread over here.
MacBook Pro 2x1.83 (Macworld demo unit) - 20x
PowerBook G4 1.67 (15" 1280x854 model) - 12.5x
PowerMac G5 2x2.0 (rev A) - 25x