i'm not sure, but just because there's an imac with the GT130, it doesn't mean it's using the nvdia chipset...
I'm guessing if it did, Apple would allow you to switch between the 9400m and the GT130, but that would be pointless on a "desktop" computer anyway...
I'd assume they only use Nvidia chipsets on their imacs with 9400ms, especially since nvidia chipsets are generally less stable than intel