Well that's entirely dependant on how well the Wii does next gen , I know the outstanding success of the Wii has everyone convinced that Nintendo has an iron grip that won't loosen any time soon but those who have observed historical trends will see that each generation is a new ball game (with momentum having a significant but not determining impact.).
We know for certain that Wii Fit 2 , Wii Sports 2 and Wii Play 2 won't suffice as the next gen nintendo experience , they'll probably want to do something even more innovative and with more innovation comes a higher level of risk (and potential reward) . We also know that Sony and Microsoft would have learnt a lesson or two and won't rely on traditional business practices in the face of a dynamic market.
It would only be logical to port PS3/360 titles to the Wii succesor because it's the only console which wouldn't have had those titles , but like I said before the viability of doing that would rely heavily on how well it does.
