How important is high rollover? NKRO requires PS/2 or specialist controllers (the Choc Mini uses the latter). USB-based keyboards are typically 2KRO or 6KRO.
High rollover is possible with membranes, but virtually every membrane keyboard in the world is 2KRO. It's a bit hard to explain, but to reduce complexity, the keys on the keyboard are wired into a grid (called a "matrix"), and pressing certain combinations of keys causes the current to flow the wrong way through the grid and this causes the keyboard to incorrectly detect extra keys being pressed ("ghosts"). "Anti-ghosting circuitry" just means putting diodes into the circuit to stop this wrong-direction current. There's a series of diagrams on this page that demonstrate the problem in detail:
http://www.dribin.org/dave/keyboard/one_html/Since you can't solder a diode onto a sheet of plastic, membrane keyboards have no diodes and therefore cannot normally exceed 2KRO. Microsoft figured that you could put resistor chains into the membrane circuit, but I'm not aware that anyone else has been taking advantage of this.
I've never heard of a NKRO in a Cherry ML keyboard either, though it's perfectly possible — it would just mean adding diodes to the PCB and replacing the controller with one that doesn't implement blocking.
