So what will this allow us to do?
Basically Wii Homebrew but on a larger scale. Like imagine stuff like an accurate N64 emulator running in 1080p which is also playable on the gamepad, nifty homebrew games/applications that could take advantage of the gamepad, game mods, or even more featured multimedia applications. Also technically speaking if people got a hold of the Wii U it means that it would be possible to do stuff like play GameCube games on the gamepad.
GC Wii Mode support has existed for a while
Check GBATemp's Wii U hacking board
I think Gr8 was implying if there was a way to implement the GameCube adapter into the Wii Mode other than in homebrew. I don't think there is way or will be, but who knows. Being able to play with GameCube controller in some of my Wii games would be great because the new Classic Controllers that imitate the GC controllers are garbage and don't hold up to the standards of them.
also hasnt there been a gcn "mod" on the wii u as well?
Yeah it's called Nintendon't, great piece of homebrew that allows you to play all of your GC games off an SD card with widescreen hacking, forced progressive scan, virtual memory cards, and even with your normal GC controllers via the adapter. It plays all the games natively using the hardware from the Wii (since the Wii is literally just an souped up GC), so there's no emulation involved.