Apple doesn't want to ship a computer with a crappy integrated GPU, because they want to ensure that users have OpenCL-capable GPUS. Intel is not offering i3s or i5s with good integrated GPUs, and Intel won't let NVidia make chipsets anymore, so in order to offer an i3/i5/i7 system with a good GPU, they need to use 3 big, power hungry chips (CPU+chipset+GPU). During the Core 2 era, Apple took advantage of the great combo of a Core 2 CPU + NVidia chipset with good integrated graphics to offer reasonable performance in smaller packages. They no longer have enough room for a 3 chip solution, so until Intel either improves their GPUs or starts playing nice with NVidia again, Apple faces the choice between using older CPUs or regressing to older enclosures and higher power consumption.

My Core2Duo 1.83GHz mini has an intel GMA 950 GPU. Are the current GPUs worse than that?

The 'GMA HD' GPUs in the i3/i5/i7M are better than the GMA 950 (or even the GMA 3000 & 4500 series as far as I know), but they're still massively inferior to the nVidia Geforce 320M or its predecessor, the 9400M. There aren't any OpenCL drivers for the Intel GMAs as far as I know, although to be honest I can't see an obvious technical reason for that in the GMA 4500 and HD. Maybe there's no point as using the CPU in the same die is just as fast. Or maybe it's because Intel are rubbish at developing decent GPU drivers - their OpenGL implementations are pretty poor.

My personal speculation is that it is do with the current lack-of-performance currently in Intel GPUs. Intel’s current i3 two chip performance is not as enticing graphically as the Nvidia’s two chip solution with a Core 2 Duo processor. I think a better question is why does Apple keep insisting on a Nvidia or AMD GPU in their current products? It’s a question that Anand Lal Shimpi has asked as well:

“Apple is up to something. For the past 1.5 years every single Mac has shipped with some form of NVIDIA graphics, standard, regardless of price. These are all G9x based GPUs with full support for OpenCL. From the looks of it, Apple is trying to broaden its install base of OpenCL compliant machines. In preparation for what I'm not really sure, but something is coming.“

(http://www.anandtech.com/show/3762/apples-13inch-macbook-pro...)

webGL?

Core i3 has an integrated GPU that benchmarks just a bit faster than the GeForce 9400M in the previous Mini, but much slower than the GeForce 320M in the new Mini/MacBook/MacBook Pro 13-inch.

The MacBook Pro 15" and 17" include Core i5/i7 but have GeForce GT 330M in addition to the intel HD graphics to provide a graphics boost. It would increase the mini cost to have a second GPU.

==== GeForce 330M (MacBook Pro 15/17)

3DMark 05 = 12503

http://www.notebookcheck.net/NVIDIA-GeForce-GT-330M.22437.0....

http://support.apple.com/kb/SP585

==== GeForce 320M (MacBook/Mac Mini Mid 2010)

3DMark 05 = 7262

http://www.notebookcheck.net/NVIDIA-GeForce-320M.28701.0.htm...

==== Intel Graphics Media Accelerator HD Graphics (Core i3)

3DMark 05 = 3096

http://www.notebookcheck.net/Intel-Graphics-Media-Accelerato...

==== GeForce 9400M (Mac Mini Late 2009)

3DMark 05 = 3002

http://www.notebookcheck.net/NVIDIA-GeForce-9400M-G.11949.0....

http://support.apple.com/kb/SP577

The i3's GPU sucks so it would need an additional discrete GPU, which would cost more and require a larger motherboard.

I guessing they wouldn't be able to get an internal power supply in there if it had to power an i3.