To discuss this rationally, (unfortunately) we need to throw out some numbers about the relationship between unauthorized immigrants and the U.S. population. As of June, 2010, the Bureau of Labor Statistics estimated our total civilian, non-institutional population as 237.7 million. Of those, the labor force is 153.7 million, 139 million of whom are employed and 14 million are unemployed in this recession. These figures INCLUDE immigrant workers regardless of status.
Based on Department of Homeland Security estimates, the U.S. unauthorized immigrant population is around 11 million. That works out to about 4.6% of total population. Under the extreme assumption that all of the 84% of unauthorized immigrants age 18 to 54 yrs are workers, that would be a maximum 6% of the workforce. So, we're not talking about huge percentages of either the population or the workforce. We can accommodate that. In fact, we're already doing it!
Unauthorized immigrants most certainly ARE paying taxes. They pay sales taxes on their purchases and user fees on services just like everyone else. They pay property taxes directly and indirectly. They even pay income taxes, Social Security and Medicare when these are taken out of their wages by the government. (Social Security is holding over $400 billion in funds that immigrants paid but can't receive in benefits.)
They also contribute to the US economy through their spending which creates other jobs. Investigations have revealed they are not sending all or even most of their earnings back to their country of origin. Furthermore, they often work in low-wage industries, which results in savings for employers and purchasers of their products.