I'd be really surprised if we get more than Intel Iris Pro this go around in the 15" chassis. Happy, but surprised.
Given there is just one model with dedicated graphics across the whole notebook lineup, it really seems they're fading it out. For most tasks, Intel's graphics are more than adequate, but as soon as you get into CUDA, CAD, or Gaming, you really need dedicated options.
If they do discrete, AMD wouldn't surprise me. They've used AMD now in the Mac Pro and 5K iMac, so perhaps AMD is willing to cut a better deal than nVidia...really hard to say.
It just nauseates me to think of spending $2k+ on a new notebook, only to get integrated graphics...All that said, I'd love to see the GTX 960M be available, so here's to hope!