It's quite simple. AMD bought ATI. If Apple wants to use ATI GPUs in their computer, then I guess they have to talk to AMD now (at least for pricing, orders, etc).
Yes. And AMD/ATI is currently working on new laptop CPU/GPU chipsets. Their first intensive collaboration. Apple and ATI have a strong link. Why not have AMD systems?
Intel's GPU solutions are pretty bad.
As someone mentioned, the x86 version of OS X works fine on AMD cpus. It's a hack, but it runs fine.