The SSD card is using a standard mSATA connector and layout. The size of the board is proprietary, but the pinout and the signal standards are maintained.
So, it's possible to build a replacement SSD for the macbook pro Retina. I expect our friends at OWC and others to start making 512, 768 and 1024GB upgrades available. Perhaps faster, since they have shown they have access to the same high-speed controllers and chips.