Yeah, I think we may see this when the Haswell models come out, I could see it being only as a build-to-order option though.
I think that having a low end 13" rMBP with a ~2.5Ghz dual core and a high end with a ~2.1Ghz quad core would probably be confusing to some. Also I think that those 35W CPU parts are relatively expensive compared to dual core.