Yup, the previous SSDs have had proprietary interfaces but the part has always been available as a field-orderable SKU by apple repair vendors (took my 512GB rMBP to 1TB this way).
It really seems like this is soldered on, esp that slide that was shown (if it is an actual shot and not a marketing mockup) that shows the logic board with the SSD highlighted, doesn't seem replaceable (seems soldered on in a BGA type form or something).
If it's a socketed interface I have no problem replacing it and having an AAPL certified repair shop ordering me the part, but I'm not desoldering the SSD
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Regardless, I'm not paying the $1,200 upgrade tax to 2TB for a machine that will be obsolete in 4 months (no DDR4, Kaby Lake on its heels?), that's just highway robbery. I'll bring external storage and sell it in 6 mos when it becomes a hunk of obsolete circuits in 2Q when Kaby Lake and 32GB DDR4 finally comes.