Apple mentions that the iPhone 3GS is the Fastes iPhone they ever made, but hides the exact processor and memory details. While we already digged out the Graphics chip powering the iPhone 3G Speed, it seems a leak by T-Mobile Netherlands confirms a 600Mhz processor and 256MB ram.
iPhone 3G had its processor running around 400Mhz (though even that was capable of running at 600Mhz). Once the iPhone 3GS goes on sale, we can expect the experts to cut through the shiny plastic body and reveal the exact details.