Updated As several of the senators taking part in a bipartisan effort to overhaul to the nation’s immigration laws appeared on the Sunday talk shows to sound an optimistic note, Senator Marco Rubio, Republican of Florida and a member of the group, offered a strongly worded note of caution: “No agreement on immigration legislation yet,” read a headline, in all capital letters, of a statement released by his office on Sunday. “I’m encouraged by reports of an agreement between business groups and unions on the issue of guest workers,” Mr. Rubio said in the statement. “However, reports that the bipartisan group of eight senators have agreed on a legislative proposal are premature.” Mr. Rubio was referring to news reports on Saturday saying that the nation’s leading business and labor groups had reached an agreement on a guest worker program for low-skilled workers — an issue that had been among the final sticking points in the overall immigration negotiations among the group of eight sena