Yes, it's pretty unlikely that Nintendo's even really thinking about the Slim. Still, it's possible. But with the sheer innovation and "coolness" of the Wii, coupled with its low price and popularity (the Queen of England owns one FFS), I really don't see it being dethroned. Plus, who can say no to Mario, Zelda, and Metroid?
And I have to think that the newly priced 360 will probably, along with the Wii, nullify any surge in sales from the PS3. Once Natal comes out, though, Sony will probably drop back to third.