Won't happen.
The iPad Mini is already half way between standard and retina. The 768x1024 resolution may be the same resolution the iPad 2 uses, but it's a smaller screen so the equivalent PPI is higher.
A retina display on a Mini would offer no benefit over the current display resolution. You don't notice the current PPI of the Mini, so you wouldn't notice double that (1536x2048). However the device would double in thickness and weight, because that's what a Retina capable device would require to operate at that resolution.
They -could- boost the resolution without doubling it, but it is unlikely that they would bother. That would entail a huge amount of work in their SDK and piss off a lot of developers (now you've got a whole new resolution to deal with, that isn't a multiple of two).
In other words, you've already got a Retina iPad Mini.
-SC