The way I see it, there won't be any physical BC but defiantly digital:
NES: confirmed
SNES: possibly confirmed
N64 unconfirmed
GCN: pretty much confirmed
Wii: given the amount of games currently being released on the Wii U eShop, it's a safe bet the they will be on the NS, assuming it has motion controls
Wii U: possibly not coming, we'll be getting ports of games like Super Mario Maker, Mario Kart 8, Super Smash Bros. For Wii U, and Splatoon, instead
GB: unconfirmed
GBC unconfirmed
GBA: possibly confirmed
DS: as long as it has touch screen support and the two screens can fit on the NS screen; left and right, I assume
3DS: I doubt the cartridges will be supported but I could see them coming to the VC, later in the system's life; similar to how the Wii U got DS VC games, later in its life
So yeah, this is how the BC will work with the Switch, AFAIK.