It's going to be changed, Nintendo X (with the X representing "cross" because of it's supposed functionality as a console and hand held) would be cool but MS would probably object.
I like NEX as well. Could be Extra, could be Nintendo Entertainment Experience, X for anything, etc. It pops and lends itself well to branding, too. Honors the codename.
My fav so far was "Annex." Goddamn that's good.
More:
The Nintendo Omnicom. Fits but prolly too Latin and/or future/cheesy. I would love some sort of call back to the Famicom name for the NX. Famicom is the best name ever.
Edit: Nevermind. I shoud have known something like Omnicom would be taken already haha.