The problem I see most frequently (even in your post DeeK) is that the idea that people coming into our country is in itself a bad thing. This is the fundamental flaw in the anti-immigration arguement.
Since the begining our country has evolved by the inclusion of new waves of people. The Germans, Italians and Irish all started as immigrant groups who were feared and loathed upon their arrival, but were eventually included into society and enhanced our culture. We wouldn’t even have labor laws had it not been for Irish immigrants who brought the solutions they couldn’t use in Ireland here where they could be implemented.
The current wave of immigrants are mostly brown people, which causes more antagonistic feelings because they seem so obviously “other” and there is the fear that our society will decline by the inclusion of these people into our culture. Same was said of the Irish.
We have the carrying capacity within this country to support a lot more people than we currenty do (not including oil- whole separate topic). We have the space, the economic ability, and the agricultural capacity to absorb and feed these immigrants. We also need them. One of the reasons that as an industriallized nation we are not seeing the decline in birthrates that England and Germany are is the immigrant population. We need the replacement rate to stay up so that programs like social security have enough workers in the system to support those drawing on it.
The deal with immigrants has been this- you come to our country, you work the crap jobs, and your children will have a better future. We get cheap labor and a source of tax revenue, they get education for their children and more freedom from persecution and abject (dollar a day) poverty. Each new generation improves their condition and increases the amount of taxes that they pay into the sytem. Yes, things should be done in their home country to improve conditions, but there will always be stratification and it is the nature of people to try to improve their situation.
Societies that are closed fail. Closing our borders and stagnating our culture with fundamentalist and eurocentric views will turn a dynamic country into a declining one. (I know there is the arguement that we are already declining- go with me on the positivist view here). Immigration is also a much better solution to population problems- it redistributes population from overpopulated countries whose resources are straining to countries that have the capacity to support those people.