Wii''s third party support will only continue to improve as the generation goes on. It's only inevitable that the highest selling console will have strong third party support. Gaming is a business and developers will flock where the money is. More developers are starting to wake up to the fact that Wii is here to stay. However, at the same time, Wii will never achieve the third party support it should be getting, ie, PS2 levels, because Nintendo and Wii's direction of gaming simply does not fall in line with what most third party dev's ideals. Until this changes, Wii will never gain the third party suport it should. Most third parties still place most of their emphasis in horsepower and graphics, while Wii pushes for new control schemes and innovation in gaming inferface. So obviously Wii still recieves weak support because most devs simply aren't accustomed to his new direction.
However, I believe the next generation more third parties will begin to conform to this new flavor of gaming, and as a result, Nintendo next console will be the biggest beneficiary.