Thomas Sowell brings up some good points about immigration in his article on immigration. Although a lot of this is stuff that does come up in discussions about immigration, particularly illegal immigration, it's worth looking at again.
Most notably, the point on using illegal (and even some legal) immmigrants from Latin America to do jobs that Americans won't do is briefly touched on. As has been said before, and will probably be said again numerous times, immigrants don't take the jobs that Americans won't do. What they do is take those jobs at a lower cost to the employer. Now, it's certainly advantageous to the employer and to those wishing to purchase his/r products for this to happen, but it needs to be done in a manner that is "fair" to potential employees. There is always a minimum rate for which Americans will provide their labor.
This rate is governed by a variety of things. For example, this rate should increase based on how hard the work is. Another factor is whether the work is there year-round or seasonal (most employment is year-round, but some farm work may still be seasonal, for example...). And of course, there is the cost of living to consider. Most of these things put Americans on the same ground as immigrants (cost of living may give some immigrants a slight advantage here).
However, immigrants (specifically, illegal ones) have one advantage over Americans (and even legal immigrants): unlike legal workers, these people aren't technically governed by such things as minimum wage laws. And the money made by these people is worth a lot more to them when sent out of the country, so lower wages are comparably good for them. Even in the case of most legal immigrants, if the worker's family is still living in (for example) Mexico, each dollar of American money will "stretch" farther when sent "home."
Thus, immigrants (and particularly illegal ones) can "outbid" American workers for certain jobs, because less money is worth more actual wealth to these people. In other words, immigrants taking Americans' jobs, not doing jobs that AMericans won't. Right now, this may actually be good for the consumers and the employers, but on the other hand it creates higher unemployment. This is in turn actually bad for the economy, because many of those unemployed people are supported via government-funded welfare programs, which require taxpayer moeny. So in taking "undeireable" jobs, these immigrants are actually providing a net drain on the economy, and hurting everybody (because of the higher taxes needed to fund welfare).