On January 29, 2013, shortly after the Gang of Eight announced their bipartisan blueprint for "comprehensive immigration reform", Sen. Marco Rubio took his case to the Senate floor and said illegal aliens that receive an amnesty through the Gang's proposal would have to "pay back taxes." Now that the bill is public, it's become apparent that most illegal aliens will not be required to pay back taxes.
Page 68 of S.744, the Border Security, Economic Opportunity, and Immigration Modernization Act, defines the tax requirements for illegal aliens that are eligible for amnesty under the bill. The tax clause states:
An alien may not file an application for registered provisional immigrant status under paragraph (1) unless the applicant has satisfied any applicable Federal tax liability.
The bill then defines the term "applicable Federal tax liability."
the term 'applicable Federal tax liability' means all Federal income taxes assessed in accordance with section 6203 of the Internal Revenue Code of 1986.
In other words, illegal aliens that have filed a tax return and owe taxes to the federal government will have to pay those taxes before they can receive amnesty. But for illegal aliens that have worked and never filed a tax return, including those that have worked illegally and were paid under the table, will not have to pay any taxes before receiving amnesty.