A few years ago they had a 15" MacBook Pro that only had onboard graphics, and then you could get the upgraded model with a dGPU. Maybe they are going to do the same here? It would be strange for them to drop the dGPU from the 15"
But what would that "upgraded model" be? Notebooks still live and die by the watt, and Nvidia is still on the same process node and same architecture. Any bump in performance/watt would consequently only be from binning, and that won't amount to much. At the very least, it can't be enough to prevent a ridiculous upgrade path, which would essentially be:
Good: Iris Pro with good compute performance, great battery life.
Better(?): Add-on Nvidia GPU with worse compute, worse battery life, all so the throngs(?) of rMBP gamers can dual-boot to Windows and watch that framerate go.
I actually wonder how optimized OSX-native Intel GPU drivers will be. I'd think it'd be relatively easy to out-optimize whatever Apple and partners have done with Nvidia/AMD GPUs so far. Apple can then go and claim a performance boost in Blizzard games and Source games (let's be honest: that'd take care of the vast majority interested parties).