The Inquirer explains why Apple chose ATI over NVIDIA for its Intel based MacBook Pro and iMac launch. It's mainly because ATI's Mobility Radeon X1600 was available and NVIDIA's G73 still not.
Apple has been well known for being rather disloyal to the graphic companies and always chose the best product available. The iMac was originally launched with Nvidia's Geforce FX5200, then it migrated to ATI's Radeon 9600/X600 and now again it is using Radeon X1600.
Apple evaluated the G73 based Geforce 7600 chips but Nvidia could not get those on time. I believe that G73 is probably going to end up faster than M56 but it is too late now as the decision and the notebook/desktop design has been made. The machines are out and selling with ATI X1600 chips only. Nvidia has to wait for the product refresh cycle and it's certainly going to take some time before that happens.