Hate to steal Apple's thunder, but the first eeePC (of all things!) was, I think, the first to do this, and many non-HDD netbooks now use this design. The MBA ones just seem to be longer, allowing more flash chips. Previous examples not only have cribbed the mini PCIe's connector but also the entire form factor (5cm or 7cm long), despite not using the PCIe protocol. Interestingly, the MBA still seems to use the PCIe connector, just the form factor has changed.
So far the various netbook designs are electrically incompatible. I don't know where the MBA one fits into all this.
EDIT: looks like they standardised the interface as "mSATA" back in June.
Hate to steal EeePC's thunder, but this has been done in embedded PCs on mini-itx form factor computers since flash storage was available, pre-sata connectors. It looked like a 1-inch nub that fits in the IDE connector. It was perfect for embedded PCs since power requirements were always very low for flash memory and an easy way to save a watt.
So far the various netbook designs are electrically incompatible. I don't know where the MBA one fits into all this.
EDIT: looks like they standardised the interface as "mSATA" back in June.