Its a good decision at this point IMO.
Its obvious that they're doing it on purpose or at least neglecting to make both cards compatible... but can you blame them?
They have a few really nice features going for them that work great on a mid range or last generation GPU. Physx and CUDA. If they allowed those GPUs to work with ATI's, then they would basically be taking away two of their primary selling points for their high end cards. If the 5870 is priced right, anyone could just buy one of those and keep their existing Geforce without losing any features.