T-Mobile Netherlands (via Wired) has let slip the formerly elusive specs for the iPhone 3G S:

• 256MB of RAM
• 600Mhz CPU

Anandtech (via Macrumors) gives further info:

Although unannounced, the iPhone 3GS uses (again) a Samsung SoC but this time instead of the ARM11 + MBX-Lite combo it's got a Cortex A8 and PowerVR SGX; just like the [Palm] Pre.

So add that to the fast PowerVR SGX graphics core, and -- to quote Steve Jobs -- it's a screamer!

Update: Apple Insider has a great run down of the technology inside the chipsets.