A new study is claiming that (duh!) caffeine is a hangover cure, at least for the headache portion of your morning-after ailments. It will not cure that overwhelming sense of regret, or satiate your craving for an egg n' cheese sandwich with a side of cold pizza.
Pat's Papers points us to this article, which states the study found "a key agent in the brain chemistry responsible for hangover headaches and... a good remedy is caffeine." Allegedly it blocks the adenosine chemical that is causing that raging morning after pain... but it won't do much for the other not-so-pleasant hangover feelings.
Researchers also spoke about the "hair of the dog" cure, noting that "more alcohol may indeed reduce symptoms, but only for a time." So your best bet* is to down some Four Loko, which has both caffeine and alcohol.