If Apple shaves 0.2" off the height and dumps so-DIMM RAM (soldering the RAM to the motherboard to save height) then probably not.
The height drop actually just throws away a portion of the "saved" DVD space to lower weight (overall system is thinner so therefore weighs less as case is smaller). Soldered RAM takes up more horizontal space than DIMMs do. The display will require a bigger battery and also will have displaced battery pack space from the lower portions due to the height reduction. Finally, it isn't just the 650M. Need space for the 650M's VRAM ( likely already soaked up by the soldered RAM. ), Display Port switch, and for additional cooling solutions.
If look at the MBPr 15" motherboard there isn't much space to be saved.
top
bottom
source iFixit teardown for what those colored boxes correspond to
The MBPr 13" will be less wide you either you need to make that board narrower and taller (in respect to the pictures above) or get rid of some of those components to make it narrower with the same height ( in respect to the pictures). In the reduction option, tossing the red box on the top picture and the orange box in the bottom can claw back enough space to make the board about 2" narrower which is what you need.
If make the board taller then making battery section smaller. That really isn't an option. Since the battery section is already smaller since saved volume off with the MBP case height reduction.
If Apple left the height alone and saved the majority of the DVD space for GPU + VRAM + Cooling it is much easier to fit those into a 2" more narrow space . They just don't seem likely to do that. They are going to be out to chuck the DVD , HDD , and Ethernet port and then toss a substantive portion of the saved space away.
Back in the day the PowerBook 12" was taller than the larger screen equivalents. Somewhat for this same reason. Too thin with a smaller screen means going to be making relatively larger volume sacrifices for fixed volume/space components like battery, RAM, GPU, etc.
This is one reason way the MBA has be relatively crippled on RAM capacity versus the MBP offerings including the MBPr (which maxes out at 16GB RAM). Likewise, trailing behind on battery lifetime also (getting better now with lower powered CPU/GPU combinations. )
