Johanns is an interesting choice, but the record for Cabinet Secretaries ascending to the WH is few (only 2, both Republicans, since 1856: Herbert Hoover-Commerce; William Howard Taft-War, and both were considered failures as President), and he may be too obscure a dark horse for the position. Johanns' successor, Dave Heineman could be just as good a candidate. He was considered little more than a seat warmer for Congressman Tom Osborne, but with Osborne's implosion (and his running mate's (Kate Witek) insanity of switching to the 'Rats after she lost), he ended up a giant-killer (at least by Nebraska standards).
A couple notes:
Several secretaries of state ascended to the Presidency--notably James Madison, James Monroe, and John Quincy Adams.
Sitting vice-presidents who won the presidency through election include John Adams and Thomas Jefferson before the Twelfth Amendment and Martin van Buren and George Herbert Walker Bush Sr. after the amendment. Richard Milhous Nixon won the presidency eight years after leaving office as vice-president under Dwight David Eisenhower.
We need a serious Republican in the 2008 race who can win both the primary and the general election. McCain (a Senator) and Romney (from Massachusetts, cannot win his home state) simply cannot win. Giuliani has major issues: questionable popularity in New York, various marital problems, a socially liberal reputation, and a big-city image that might not appeal in the farm states.