The flash memory is still PCIe-based, and remains, thankfully, removable—at least for now. Popping it out for inspection, we find:

Samsung S4LN058A01 PCIe 3.0 x4 AHCI flash controller

Samsung K4E4E324ED 512 MB LPDDR3 DRAM

8 x Samsung K9LDGY8S1D-XCK0 16 GB flash storage (128 GB total)

Apple tells us this bad boy "...features faster flash storage that is up to two times faster than the previous generation."

We did a little benchmarking, and found that the claim is valid. Nice one, Apple!