Several sites are pointing to a blog with speculation that AMD's recent acquisition of ATI may mean that Apple will drop ATI.

Will Intel allow Apple to continue to working with ATI on graphics after the company becomes wholly owned by rival AMD? I think that we'll probably see Intel gently "suggest" that Apple switch to another vendor for graphics technology in 2007. Once the dust settles I bet that all Macs from here on out will ship with graphics from either Intel or NVIDIA.

This remains entirely speculative.