The iPod touch 2G had a 533mhz processor (downclocked from 600mhz but w/e) and was 'bumped' to 600mhz in the iPod touch 3G- not 2x but around 50%.
The iPhone 3G had a 412mhz processor and was 'bumped' to 600mhz in the iPhone 3GS so it's closer to 2x.
Errr... The Iphone 3GS has a completely different processor then the 3G did, it's the ARM Cortex A8 vs ARM 11 Core on the older 3G. It's what allows Apple to claim 2x the performance of 3G processor, without doubling the clockspeed.
Hopefully the 3G Touch also has the ARM Cortex A8 at 600MHZ like the 3GS, we will see...
Going from a ARM 11 at 533 MHz to an ARM Cortex A8 at 600MHz isn't as much as a boost. Apple's math is right if we accept their claims that the 3GS is 2x as fast as the 3G.