Is the retina Macbook Pro's SSD soldered into the logic board?
No - The SSD card is a discrete component on the new retina MacBook Pro and can be ordered as an Apple service part. Time will tell if an aftermarket supplier decides to supply parts for that model.

Even though it's not soldered, it still is not a user-servicable part.

As ArsTechnica reports:


  While Apple uses the latest SATA protocol, the connector is physically incompatible with either Mini PCIe or mSATA.