I've heard one of the chief complaints about the illegal immigration problem as being the lack of taxes being paid and there is some truth to this. Although it may have other problems and issues depending on one's personal financial situation, the propsed Fair Tax Act which would eliminate the current income and social security tax along with the various capital gains, corp., estate and other various income taxes and replace it with a national sales tax would for the most part eliminate this problem of purported non-payment of taxes.
As a libertarian I believe that globally and by unalienable right we should be able to transcend national borders at will. However, when you have a type of infrastructure that we have here in the US and likewise for Mexico based on what I've read at the earlier link I posted and at other sites, I can understand the need to have some control and ability to moderate access to that infrastructure. Mexico's rules that I read seem prudent enough and my understanding is the current US rules are likewise similar. Canada may as well be near the same. You know, you would have thought that within NAFTA that this issue got some mention as the heart of any business to begin with is the labor to drive it.
It's also funny to me that with all the union hesteria about all the jobs leaving the US and going south that we'd have so many people annually making the hard trip north and south all the time to find work. Like many, many things we see and here, this may be vastly overblown as well.
JMO.