I was wondering that myself. Given Apple's history, I'm pretty certain that vRAM size is a firmware issue, thus you'll be stuck at 128 mb vRAM for the lower-end Mini, while the upper-end will give you 256 mb vRAM. That would be too bad, because one could buy the low-end and install their own 4 gb RAM and a 500 gb hard drive for less than the cost of the upper-end Mini.
So says Apple, so say we all.