Xeon Phi may well cement this further. Knight's Landing, due in 2015, is going to feature 72 Atom cores (288 threads) with AVX, socketed in a standard Xeon motherboard.
In situations where your problem domain doesn't fit comfortably within the memory restrictions of a GPU, or porting a legacy code base difficult, this could be a very interesting option.
It is a non-starter for the successful commercial renderers because the majority of their market already has render farms with standard CPUs. Also studios generally use a mix of tools and if only one of them is optimized for Xeon Phi's that isn't enough of a motivation to spend the money on Xeon Phi's.
It is generally a no go in the mainstream rendering market, although cloud-based renderers can use specialized solutions.
You may be able to accelerate some steps in a GPU, but rendering usually involves "a lot of data", if you add all resources for a given scene it might be on the order of gigabytes
This is not like bitcoin mining where you have to do a lot of math in a small batch of data.
- CPU renders typically might use 32Gb memory (or more).
- most rendering is done on a render farm which won't have GPUs (unless you maintain a gnu render farm).
There are good reasons why commercial renderers are avoiding the GPU, at least for now.