This CQ Weekly article indicates that Sen. John McCain might pull his support for the immigration bill he and Sen. Edward Kennedy pushed last year over a provision which would apply the Davis-Bacon Act, which requires that workers receive local prevailing wages and benefits, to certain provisions in the draft related to guest workers (and thus, actually attract U.S. workers to the jobs conservatives complain illegal immigrats are taking). However, this provision was already included in the Comprehensive Immigration Reform Act of 2006!
Republicans hate the Davis-Bacon Act in general (President Bush suspended it for reconstruction workers in New Orleans after Hurriance Katrina), but McCain didn't seem to have a problem with it last year. Maybe it's just a convenient excuse to no longer support immigration reform that the conservative base of the Republican Party overwhelmingly despises now that McCain is running for their presidential nomination? He's already pulled his support for further campaign finance reform... but perhaps I'm just being too cynical.