I think you are right. Portland signed 3 Argies this off season. Chivas picked up a player on loan from Argentinos Juniors. SKC too a Brazilan on loan. Our Brazilans. Montreal have a few South Americans and Piatti from San Lorenzo incoming this summer IIRC. Orlando look like they will have a hkeavy Brazilan influence next year.
Dallas and Vancouver added other South Americans.
While it's always been a market thats been tapped by MLS, I think the level/amount is increasing.