The Guardian has an interesting article on and interview with the Pixies that begins to explain how and why they got back together (even if they are just touring as of now). It also provides a nice, digestible history of how they came to be in the first place.
For many years, a Pixies reunion was about as likely as a full Beatles or Clash reunion. They were rumored to hate each other that much. So what happens, Frank Black makes a verbal slip up (or an unguarded moment of honesty) and the whole thing gathers steam from there.
However it happened, their show at The Fox last October was probably the best show I saw in 2004. And 2004 was a good year for shows.
UPDATE: The comments mention The Smiths as another band about as likely to get back together. Well, here is why it will never happen. If I were a teenage girl, I'd probably be all over it though.
NP: "Your Sickness" - Pinback