Robert Samuelson ("The Real Economic Report Card") brings up a point that ought to be obvious but isn't made more often: unskilled immigration makes economic statistics such as poverty levels and the number of those without health insurance worse. It seems like common sense that if you import more poor people whose likely jobs (landscaper, busboy, non-union construction worker) usually don't include health insurance, you will end up with more poor, uninsured people in the country than you would otherwise. Perhaps the reason this point isn't made more often is that immigration is one area where there currently is no significant policy difference between the major party presidential candidates.
It makes some sense, from a political perspective (if not from an economic perspective), for Democrats to be in favor of increasing unskilled immigration. If unskilled immigrants become citizens, they are more likely to be net recipients of government benefits, and thus more likely to vote for the party that favors more progressive taxation and more generous spending on social services. Unskilled immigration makes less sense politically for Republicans, but it makes economic sense for certain Republican constituencies, e.g., the small business owners who hire the busboys, landscapers, etc. From the perspective of these small employers, the GOP offers the best of both worlds: support for cheap labor and support for pro-business policies such as lower taxes. Of course, over the longer term, it's hard to see how an embrace of unskilled immigration won't consign the GOP to permanent minority status, unless somehow these immigrants are never given a chance at citizenship, which seems unlikely.
The real estate bust, by drying up the demand for residential construction laborers, seems to have put this issue on the back burner for now, which helps John McCain, since his stance on immigration is one of the areas where he is against his base.