In fact, they both use nVidia’s MCP79 chipset, that is described like that:

The MCP79 chipset is a single-chip solution for small form factor notebooks. There are six versions of the MCP79. All the versions include a DirectX 10 GeForce graphics core which supports Shader Model 4.0, NVIDIA’s VP3 video processor, Hybrid Power, Hybrid SLI, and Hybrid Performance.

All versions of MCP79 includes a DirectX 10 GeForce graphics core (GeForce 9400M or GeForce 9300M). So this is disabled by Mac OS X on the new iMac instead of using it for OpenCL, as GeForce 9400M is far faster in real-world OpenCL processing than Radeon 4670HD due to it’s archiecture (tehnically: support of unified read-write in GPU SM shared-memory).

I don’t understand why Apple is using a chipset that is great for OpenCL processing, with peak performance-level of 40 Gflops (Core2 Duo 3.06Ghz peak performance is 24.28 GFlops, 3.33Ghz version reach 26.64), and enable good use of OpenCL, aggregating the computing power of CPU + GPU to sometimes double the performance-level of CPU-alone.

This is truly disturbing, as the same chipset stay with GeForce 9400M active on a MacBook Pro when display is handled by GeForce 9600M GT, and both GPU are used by OpenCL!

Why Apple act so strangely with it’s hardware, paying for it, putting it on the computer, making the consumer pay for it finally… And disabling it!