I agree with you and probably so do most archaeologists. Some contact perhaps did occur, but contact and diffusion, as you note, are not the same thing. To say that Africans or Polynesians or Chinese or Japanese or Apaches, for example, drifted into Mesoamerica may or may not have happened. We simply do not know despite the claims of many of those who drift into and out of this news group. There is absolutely no hard evidence in any museum or material gleaned from any archaeological site to even suggest it happened. This doesn't mean it didn't happen. It simply means we haven't found it yet. Many of you say that we who do the digging are "covering up" for one another. This is the most absurd statement I have ever heard. What in god's name would cause you to conclude that as an archaeologist I would spend months away from my family in some remote place living under less than ideal conditions and eating things I really would prefer not to and finding things in my bed and clothes I would rather not find so I could say to my colleagues, "See--you're right!" Hell, I would die for something that I could use to generate headlines and get me more bucks for research. It sure as hell works for the "dinosaur diggers". If I could just find one "bone" you would read about it in tomorrow's New York Times and see me on CNN.
On the subject of diffusion. I don't understand why so many of you are so insistent that it occurred. All of the civilizations being proposed as contributors and recipients are so radically different that any infusion of new ideas would have been as earth shattering as the encounters in the movie "Independence Day". This would have been aliens meeting aliens. If this contact had occurred in Mesoamerica it would have been broadcast from the monuments right up to the end. The books of the "Chilam Balam" would have talked about descending from the ancient ones who came from across the seas. There are a number of other historical native documents whose very foundations involve "who begat whom" to validate land claims. I know if any of these peoples could have pumped up their claims of legitimacy with trumpets of diffusion, you would see it in their writings.
Today you can see number of peoples in Mexico with "negroid" or African features, not because of some early migration, but because the Spaniards brought African slaves to Mexico. At the time the Polynesians are being suggested as contributors, Mesoamerican civilizations had already developed beyond the formative stages. All of their belief systems and architectural traditions were pretty much in place. So were their weapon systems and combat modes. Pottery production with minor variations was established and did not radically change until the Spaniards introduced the potter's wheel. Weaving and textiles had been invented and the plants which produced these fibers were already being selectively propigated. House building methods date from the earliest times. Peasants living in Mesoamerica today still live in the kinds of homes their ancestors were building 4000 years ago. The Chinese probably did have an impact on Mesoamerica, but this occurred some time during the migration from Asia to the new world. The people who left Asia had belief systems of some kind. They did not leave their brains "at the border" when they ventured into the Americas. When they left and where they left from and the routes they took are now known to us today thanks to mitochondrial DNA research.
There was diffusion from South America into Mesoamerica. Pottery production technology is one example. And, there was diffusion from Mesoamerica to other parts of North and South America. One only has to look at the distribution of maize/corn for evidence. Items classified as "elite goods" which appear to have been manufactured in Mesoamerica can be found in North American sites. That the peoples of the southwestern United States together with the Hopewellian and Mississippian cultures had contact with Mesoamerica is pretty obvious. So what you have here is a line of contact going back for thousands of years. None of this contact radically changed the cultures involved. Diffusion by its very nature suggest radical input. This radical input did not occur. Most, if not nearly all of the people who ever lived in Precolumbian America never ventured more than 10 kilometers from their place of birth. You find this all over the area today. There are still people in central Yucatan, for example, who don't know what lies 25 kilometers beyond their village.
If you want to argue that contact occurred, that's your right. We can also debate how many angels can dance on the head of a pin, too. Both would be just as producive. If just one of you can show me a single artifact which proves Precolumbian contact from beyond America, I will, in the words of Andy Warhol, insure you get your 15 minutes of fame. I guarantee it.