I can confirm that they do not offer 2-3x "real world" performance improvement, Steve took care to mention that the processor is faster but the hard drive RAM etc are the limiting factors of speed in the keynote speech (a fact that most people do not mention when they dis the iMac)
Also WoW runs at near DOUBLE the FPS on the intel imac than the old PowerPC iMac ( I have the beta of the intel binary installed on one of the machines at work)
