The funniest thing is that Jobs said that the processor is dual core, and that it's 2x faster. Now this seems to imply that 2x cores = 2x faster, which is absurd, if they didn't increase the clock speed there is no way they could get a definite 2x speed up. The problem with multiply cores is that they have to access memory among other shared resources which really hurts when one core blocks out the other one. Modern…
While you are correct that simply adding a core would not result in a 2x speedup you missed the fact that there are other significant differences between the A5 SoC compared to the A4.
* DDR2 vs DDR1 memory (1066 vs 800Mhz)
* possibly dual-channel i.e. 2 64-bit buses instead of one but this is hard to get solid info on atm.
Even so the speed up probably wouldn't be exactly 2x, but it would be close in some applications and make his claims a lot more reasonable simplification for the target market.