The internal storage capacity on a Retina MacBook Pro or MacBook Air can be augmented with an SD card. The problem is, you’ll have half an SD card sticking out the side.
A MicroSD card would fit entirely inside if only there were a half-height SD-to-MicroSD adapter available. There are a few products manufactured to solve this problem, but only one is currently shipping that I’d recommend. At $20 it’s overpriced, but probably better than making your own.
Another half-height adapter is made by Adafruit. It’s thick and unyielding by comparison and might be a little rough on the internal connectors, so I opted for the gentler solution.
Here are some pictures of the MiniDrive as shipped. Clearly it’s a limited run, but I’m satisfied with it. When the original Nifty MiniDrive is available, it looks to be more expensive and much nicer. The current MiniDrive as pictured seems more like a prototype — it could have a better extraction mechanism than the simple yet effective piece of Scotch tape.
If you do get an SD adapter for your MacBook, use a UHS MicroSD card for best performance.
Update 2014: The NiftyMiniDrive and the MiniDrive functionality is going to be crushed by the Transcend JetDrive.