Maybe, maybe not.
Arguments for maybe:
1) Most obviously, it is selling at a better pace than the PS2,
2) Wii is tapping into a new market. We don't know how big that market is, and how much is left to be tapped.
3) Still hasn't had a price cut (and we all know it could drop to $150 tomorrow without really hurting Nintendo a bit).
4) That third party support is just about to start hitting. If Nintendo has been planning smart, they should be able to keep up a steady supply of solid titles for the next 2-3 years (Mario Galaxy 2, Zelda Wii, Icarus, etc).
Arguments for maybe not:
1) The 360 and PS3 are putting up stronger sales than past second and third place finishers (N64, GCN, XB, DC, Saturn, etc), and at least one of them has seen what Nintendoes and is doing its level best to strike back hard with this current generation.
2) This is the first time that a home console has been released with relatively low-tech. Nintendo may pull the plug after 4-5 years, which will make it hard to topple the 7 year run of the PS2.
3) Again, we don't know how much of that new market is untapped. It may not be sustainable for the whole generation.
4) Those third party games may suck, or gamers may not buy them, and developers may choose to leave the Wii as Shovelware Central.
5) It is debatable how much Nintendo "gets" the core crowd (as seen by their belief that Animal Crossing is a "core" title). And they have a bad habit of leaving their top franchises to rot after their first release of the generation.
