In any case - it seems as the third world also industrializes they will also shrink population wise.
Let us not forget the great Malthusian error.
I don't expect the increase in Hispanic population here to have any where near the cultural impact that the muslims will have in Europe (if they allow it) - simply because the differences aren't as great.
I too believe most people will simply reproduce less as they become more wealthier.
As for whether having less people is good or bad, depends on your preferences - environazis would love it, economically it wouldn't be so hot... but think of all the additional space we would all have!