There is just a pipe between the 2 sections of the chip, so communication between the 2 is exactly the same as if it was on a different chip, but it's built as one.
AMD iGPUs are the same. Actually, not quite - Sandy/Ivy Bridge share the L3 cache with the GPU, while AMD doesn't have that ability. Unless Trinity adds that, which it might.
Eventually they will get better integrated, and you'll probably see things like the CPU transferring floating point operations onto the GPU hardware, but i don't know how far out that is.