Just speaking colloquially, I can see it ending up third this generation.
Again this is just my experience so take it with a grain of salt. All my friends have owned Wii consoles at some point in this generation - very few own them any more (the ones that do are mostly casual players who have copies of Just Dance 3 and Wii Sports Resort). I don't think they're likely to buy a Wii-U, but I may be wrong.
People I know who like Nintendo - myself included - aren't thinking about getting a Wii-U at the moment. My best friend is a big Nintendo fan and I've recently convinced him to join me on the dark side (PS3) - he says he has no intention of getting a Wii-U any time soon.
I was HUGELY looking forward to playing Pikmin 3, and whatever 3D Mario game they come up with next - but again, it's not enough to make me buy the console. I'm happy with my PS3 for the next few years and I'll probably upgrade to PS4 once enough first-party games for that system come out.
My other friends are pretty loyal to Microsoft, and they're old Nintendo fans. About 8 or 9 of them did the exact same transition - N64 -> PS2 -> Xbox 360, and they never looked back. They won't be buying a Wii-U, they'll be getting a nextbox.
I can see Sony taking another blow next generation, but hopefully they'll keep the PS3 crowd they have now. I do know quite a few people who used to have 360's that made the jump to PS3 after their umpteenth RROD, I don't know if they'd ever go back to Microsoft. But I'm not really sure what market Nintendo are aiming to attract with the Wii-U, and as such I can see them ending up third. I'll probably be wrong, who knows.







