Both the Wii and DS passing the PS2 in sales is currently possible, but it still depends on a crapload of completely unknown factors.
The biggest one is probably how lasting the appeal of the Wii will be. It's possible for people to grow less interested in the Wii if HD adoption rates soar, or developers get really unimaginative. On the other hand it's at least equally possible for graphics to loose a good bit of their importance as people simply don't care if their games are HD or not. At least the more casual part of the market that more than half of the PS2's userbase consists of couldn't care less.
Another factor is how quickly and completely developers migrate to the Wii. So far it looks very good, but I suppose it's theoretically possibly for companies to keep their best teams making their huge epic games on the other consoles where they don't make money, and only put ports and minigames on the Wii to stay afloat economically. I'm sure everyone understands how stupid a business model that would be.
It's also possible for Nintendo to get greedy and release a new console early on that is not counted as another model of the Wii, but rather a new machine entirely. To me this would mean that the games for the new one don't work on the original Wii, but I don't think everyone agrees.