This could very well be a factor. Also, a few weeks ago I saw Dr. Condoleeza Rice being questioned about our "porous" southern border. She acknowledged it is a security problem but cannot be completely closed because that would impede "commerce." That is congruent with the theory that an impending "North American Union (NAU)" is the reason the invasion has been allowed.
My theory of why Mexicans continue to stream northward is simple greed. I have traveled in Mexico and it's not a bad place. The cities are mostly clean and the residents well-fed and well-groomed. However, El Norte offers the quick buck; the macho man can send back mucho dinero to buy things for the family, impress the rest of the village. As I said, though, it's a theory. Many years have passed since I traveled there.
I've heard that they'll even go into debt here, so they can go back and visit and show off a nice car and clothes and brag about how well things are going for them in the States.