I think the new iMacs are also getting Nvidia graphics! The problem is Nvidia doesn't have a "kepler" GPU for the high end 27" iMac. Nvidia currently has 2 "kepler" GPU's the GK107 and the GK104. The GK107 is used in the GT650m and the GK104 is used in the GTX680m The problem is that the GTX680m is probably to power-hungry for the iMac So Apple has to wait for the GK106, which hasn't been released yet. The GK106 will be used for the GTX660m and will be around twice as powerful as a GT650m and would be a nice match for the new iMac. Rumored release date is "late" summer.