There's no GS (Graphics Synthesizer) chip in the 40GB or the newer 80GB, so no PS2 backwards compatibility. The old 80GB had the chip, but no EE (Emotion Engine) which was the other half of the PS2 chipset and was emulated for backwards compatibility. Only the early 20GB and 60GB models have both the GS and EE and full hardware backwards compatibility. (BTW, speaking of US models here.. I think the 60GB Euro model was missing the EE also.)
So, to answer your question. Until Sony release an emulator for the GS you won't be able to play PS2 games.
