The Macbook Air's SSD can be manually upgraded.

The RAM of the Macbook Air and retina MBP however are soldered into the logic board.

Question

Is the retina Macbook Pro's SSD soldered into the logic board as well?

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.

It appears that the SSD is removeable. Notice the connector on the right and the single screw on the left.

The iFixit teardown has been completed for the Retina MacBook Pro and confirms that the SSD is removable and its a new proprietary design and form factor.

The picture is provided by Apple and shows the bottom of the laptop and reveals the internals: http://www.apple.com/macbook-pro/design/