ADDED: Here's the relevant material from Scalia's dissenting opinion:
It has become clear that federal enforcement priorities—in the sense of priorities based on the need to allocate “scarce enforcement resources”—is [sic] not the problem here.
After this case was argued and while it was under consideration, the Secretary of Homeland Security announced a program exempting from immigration enforcement some 1.4 million illegal immigrants under the age of 30.
If an individual unlawfully present in the United States
“• came to the United States under the age of sixteen;then U. S. immigration officials have been directed to “defe[r] action” against such individual “for a period of two years, subject to renewal.” The husbanding of scarce enforcement resources can hardly be the justification for this, since the considerable administrative cost of conducting as many as 1.4 million background checks, and ruling on the biennial requests for dispensation that the nonenforcement program envisions, will necessarily be deducted from immigration enforcement. The President said at a news conference that the new program is “the right thing to do” in light of Congress’s failure to pass the Administration’s proposed revision of the Immigration Act. Perhaps it is, though Arizona may not think so. But to say, as the Court does, that Arizona contradicts federal law by enforcing applications of the Immigration Act that the President declines to enforce boggles the mind.
“• has continuously resided in the United States for at least five years . . . ,
“• is currently in school, has graduated from high school, has obtained a general education develop ment certificate, or is an honorably discharged veteran . . . ,
“• has not been convicted of a [serious crime]; and
“• is not above the age of thirty,”