Sony has a knowledgebase article that explains which three versions can play PS2 games.
Models since the MSG:IV bundle had neither the CPU nor the software emulation of the CPU required to play older games. It can still play most PS2 games, but support isn't as good as the earlier models with the actual EE CPU. The 80GB Metal Gear Solid IV bundle had the GPU as well, but replaced the Emotion Engine chip with software emulation of the chip. These models can play almost all PS2 games. This CPU/GPU combo was put in all PS2 units and in the first variants of the PS3 (the NTSC 20GB and 60GB models) to make them backward-compatible. PS2 games were created to work on Sony's Emotion Engine CPU and their Graphics Synthesizer GPU. They cannot run on the CPU designed for the PS3.
