AMD (formerly ATI) "discrete" graphics silicon is now offered in the larger, more powerful MBPs. As in previous MacBook Pros, the discrete graphics chip is only fired up when heavy lifting is needed. When power savings is paramount or high-end graphics processing is not necessary, the system defaults to Intel's graphics.
Apple is said to be sticking with graphics silicon built directly onto the Sandy Bridge processor for the 13-inch MacBook Pro.
Read More [via Marcus]