[Corrected 12-Feb-2007. Paul reminded me that Humphrey was the sitting VP in 1968. Oops.]
Apropos of nothing, it occurred to me this evening that 2008 will be the first U.S. presidential election since
1968 (Nixon vs. Humphrey) 1952 where neither the sitting president nor the sitting vice president is running. If you broaden the definition to include past vice presidents, you have to go all the way back to 1952 (Eisenhower vs. Stevenson), assuming Gore decides not to run.