One thing I have noticed is that there is an element of the growth of the Wii's userbase which is very similar to how pyramid schemes work ...
One person (possibly a 'Hard-Core' gamer) in a social group buys a Wii with a couple of multiplayer games, and then invites their friends over to play their Wii; there friends are made up of people similar to them, although there is much more diversity in game playing habits. Now, some of these people at the gathering enjoyed the Wii enough to buy one of their own, and they end up holding gatherings where people play the Wii on their own; to a certain extent you can see this behavior on the sales charts as a large portion of the best selling Wii games have excellent local multiplayer gameplay.
There are two things that are happening at the same time which (if they continue) can lead the Wii to unprecidented sales levels. First off, the combination of word-of-mouth and direct experience with the Wii represents a level of "marketing" that is far more valueable than any marketing you can buy; a company like Sony or Microsoft could invest Billions of dollars in marketing campaigns every year and not get nearly the interest in their product that the Wii is currently receiving. Secondly, the Wii is changing how people relate to videogames; this may seem like an unusual thought to some people, but before the Wii came around few people (outside of "Core" gamers) had get-togethers specifically to play videogames.
I have peronally seen a change in people's social habits where playing a card-game or a game like scrabble/yatzee has been replaced by playing a game on the Wii; being that this is nearly 18 months after these people were exposed to it, and these Wii games are still as commonly played as they were soon after the Wii launched, I think it is fair to say that this represents a long-term change in behavior.
At the same time, don't think that this massive dominance in the local-multiplayer setting means that the Wii can't perform well as a single player device. There have been several single player only games (Resident Evil 4, The Legend of Zelda, Super Mario Galaxy, Metroid Prime, and Sonic and the Secret Rings), as well as games where the single player gameplay is a major selling factor, that have performed amazingly well on the Wii. This (to me) indicates that people are still looking for solid single player games on the Wii even though its social approach may be what is driving more sales.
As Wii Speak lets Nintendo turn online-multiplayer into a social (rather than anti-social) way to play games, third parties start producing high quality local-multiplayer games, and the combination of high-hardware/software sales and low development costs lead to the Wii having an excellent library of conventional single-player and online-multiplayer games it should be able to (at least) maintain its current sales pace for the next several years.
Whether the Wii becomes the best selling system of all time mainly depends on when it is replaced by Nintendo (which depends on when Sony and Microsoft release their next systems).