I'm surprised that nobody has mentioned the possibility that it's meant to be a shorthand expression.
NX -> Annex
An annex is an extension on a building. If Nintendo intend the NX to sit alongside the Wii U and 3DS (and I suspect they do, kind of like how the DS was initially intended to sit alongside the GBA and GC), then it makes perfect sense - it's not a replacement of a current system, it's a new system to act kind of like the "third pillar".
We know it's "a dedicated game system utilising a brand-new concept". This makes me think that it is, indeed, intended as an "annex" to the current gaming scene. I'd be tempted to suggest that it would be a VR system... but I don't think they'd call that a "brand-new concept". If we assume that it's in the same category as the DS's touchscreen or the Wii's motion controls, then we'd be expecting something that changes "how we play". But I can't really think of what it could be, right now.
One other possibility is that it could be a kind of reference to Star Trek, with "NX Project" - http://en.memory-alpha.org/wiki/NX_Project
In that case, I'd suggest that it was more along the lines of the SNES -> N64 shift, that took us from 2D to 3D. Breaking some sort of "limit".
It would be pretty cool and fascinating if the "tech acquisition" Nintendo made was Euclideon's engine, and "Project NX" was a new system designed around that engine. But it's probably not that.







