This information was grabbed off of a T-Mobile Netherland's Website.
Although this new CPU is much faster that the current one in the iPhone 3G it is more likely that the PowerVR SGX graphics chip has a hand in the increased speed too.
Read more here.
Apple's being pretty cagey with the hard specs of the iPhone 3G S, but apparently T-Mobile Netherlands didn't get the message -- it's just posted up a specs page listing 256MB of RAM and a 600MHz CPU. That's up from 128MB and 412MHz in the first-gen and the 3G, and it's basically exactly what was rumored.