Looks like DaringFireball.net nailed it: the S in iPhone 3G S stands for speed. Faster processors (we're guessing both CPU and GPU in line with new ARM and PowerVR chips), new OpenGL ES 2.0 implementation (no word on OpenCL yet, which leverages GPUs and CPUs).
HSPA cell downloads are also boosted up to 7.2 Mbps, where available.
Missing in the speed-boost department, however, was any word on support for 802.11n Wi-Fi according to the Tech Specs.
Guess we're waiting on fourth gen for that?