According to this Washington Post article,

"A $388 billion government-wide spending bill, passed by Congress on Saturday, was stranded on Capitol Hill yesterday, its trip to the White House on hold as embarrassed Republicans prepared to repeal a provision that could give the Appropriations committees the right to examine the tax returns of Americans.

Top GOP lawmakers disavowed the provision, expressed surprise that it was in the bill, and blamed both the Internal Revenue Service and congressional staffs for incorporating it into the omnibus spending package funding domestic departments in 2005."