The Chicago Tribune has an article on who may or may not be in for '08:
The list of prospective candidates "is almost certain to include senators, a few House members, a governor or two and a former mayor. It also could include other politicians who have taken care to harbor their presidential aspirations with discretion."
"One early version of the 2008 Republican roster includes Sens. Bill Frist of Tennessee, Chuck Hagel of Nebraska, John McCain of Arizona and George Allen of Virginia. The list also could include Gov. Bill Owens of Colorado and Gov. George Pataki of New York. Former New York Mayor Rudolph Giuliani also has hinted at his presidential ambitions."
"In a sense, 2008 will be a referendum on President Bush's past and on his second term," said Dante Scala, an associate professor at St. Anselm College and a fellow at the New Hampshire Institute of Politics. "This first year or two will determine whether Republicans think it's smart to run as the president's successor or someone who wants to correct the path of the party."