Well I have good news and bad news here:

• First the good news - Apple's custom SSD blade drive is a replaceable unit so its possible to swap-out.
• The bad news here is its too soon to say when 3rd party replacements will hit the market as this system is just too new.
• The other bit of bad news is largest SSD so far available is still very expensive and only 920GB in size. I would recommend you follow this site: Transcend they appear to be aggressive on offering options for Air & Retina systems.

I end with a bit of good news here: Intel and Micron’s 3D NAND promises SSDs “greater than 10TB” so in a few years we should start seeing lower cost SSD blades and larger ones as well.

Good News! OWC has just released their SSD's for the Retina systems!

Here's the details: MacBook Pro with Retina 13" & 15" Late 2013 - Current