A year ago, a dual 2 Ghz PowerPC processor machine was half again as fast as a dual 3.6 Ghz Intel machine.
But now a single 2.1 Ghz PowerPC processor machine is half the speed of a dual 2 Ghz Intel machine.
Assuming that two processors offer a 20% speed increase over one and that a 3.6 Ghz pentium processor is twice as fast a 2.0 Ghz pentium processor (ref Tom's Hardware), it appears that CPU benchmarks run almost four times faster under OSX than under XP.
Either that or someone's cooking the books. Apple really ought to take down that first page.
That 2GHz pentium (really Intel Core Duo) is based on an architecture that's very different from the 3+ GHz Pentium 4. It's much more similar to those 2-2.4GHz AMD 64s that have been very competitive with those 3+ GHz pentium 4s for a year now even though they consume a lot less power than the pentiums. So the fact that a 2GHz Core Duo is competitive to a 3+GHz Pentium 4 is not a suprise.
A 2.0GHz Core Duo is about the same speed as a Athlon 3800 X2 (AnandTech) and an Athlon 3800 X2 is noticably faster than a 3.2GHz Pentium 4 (again AnandTech)
Also as Jobs said at the keynote, this time they used optimizing compilers on the Intel side :-)