They won't. The idea is to provide an incentive to Mexicans who want to work here to do it legally. This means they would have recourse to complain about not getting SS or min. wage or others rights granted domestic workers. This means business will no longer have a co-conspirator in crime which would force a lot of them to pay taxes. And of course this should make American workers more competitive w/Mexicans.
Of course, the whole idea can be miserable failure but even if it is I can't see how the situation could get worse.
We'll have to see what happens but again, I have hard time believing many illegals will change the way they're living at the moment.....get paid in cash (no taxes), free health care, housing and food assistance, use fraudulent ID's to collect extra benefits, retail establishments going more and more to dual language packaging so they don't have to learn English, etc. and the beat goes on.....Bush is dreaming if he thinks he's going to change the status quo.