Rumor has it that the upcoming Ivy Bridge IGP will support OpenCL.
"The Ivy Bridge GPU adds support for OpenCL 1.1, DirectX 11 and OpenGL 3.1"
See
http://www.anandtech.com/show/4830/intels-ivy-bridge-architecture-exposed/5 for more details.
As for why it's so difficult to mine on IGPs, it's because they weren't designed to compute. AMD and Nvidia specifically gave the hardware computational abilities. That's why you cannot mine with an old ATI 3870 or Geforce 7900GTX. They came out before APP and CUDA.
Well, reviews are up for Ivy Bridge, but it looks like the OpenCL driver is still MIA. It might be awhile before we get any mining benchmarks for Ivy Bridge.
At what point would IB (and Haswell) make CPU mining relevant again? Or, more troubling, would it make botnets that much more dangerous? If IVB was good for 10MH/s and Haswell GT3 was good for 20MH/s, it could really add a new wrinkle to the casual mining market. Even though 10-20MH/s might not be a lot, when you're shipping 150M desktop PCs a year even a small percentage of users mining would make a large impact on the network.