All you have to do is go here:
and Register. After an email confirmation, you sign a Non-Disclosure Agreement. And... that is it. No wait times, no phone interviews. You get access to the same Wii U and 3DS SDKs as any other licensed Nintendo developer. You can purchase development hardware! And a new forum was started up for developers to discuss things.
I've been a Wii U developer for a few years now, but this is more amazing. Now I'm a 3DS developer, too! :D
No NX stuff probably because the SDK/devkits are still deeply in flux and only internal teams/second parties/damn close friends have access to proto kit. Personal guess as to when everyone else can access materials: probably after the NX is announced.
(inb4 use this for homebrew: no, bad user, read the NDA)
(inb4 use hax to make a retail unit a devkit: it's not going to be as good as a real devkit and also Nintendo will stomp your ass, so use kickstarter or gofundme to get a real kit)