The Pew Hispanic Center has sponsored two polls in Mexico this year on desire to immigrate to America. The results are eye-opening. When asked if they would be inclined to go to work legally in the U.S. in a temporary worker program, 52% of Mexicans in February and 54% in May said "Yes." With 106 million people in Mexico, this translates to 56 million people (assuming they bring their underage dependents, which the Bush plan would allow).
Unfortunately, Pew didn't ask how many would stay on illegally in America after their time ran out.
As for the U.S. running out of Mexican illegal immigrants, as Bush pollster Matthew Dowd claimed would happen in his recent New York Times op-ed, 21% of Mexicans said in both Pew polls that they would be inclined to go work and live in the United States without authorization. That's 22 million people. (And about twice as many Mexicans would be inclined to emigrate to America if they could do it legally.)
Robert Suro of the Pew Hispanic Center noted:
In other words, about one of every eight adults born in Mexico now lives in the United States. In the PHC surveys, 46% of Mexican adults in February and 37% in May said they have a relative living or working in the United States. Those Mexicans who have already migrated serve as a draw for future migration. Respondents with family members here were more likely to say they would migrate if they could (52% in February and 54% in May) than those who do not have relatives in the United States (33% and 40%).