CVG just had to go and interview Jonathan "Still Alive" Coulton, the man who created the ending theme to the Orange Box's Portal. In the interview, Coulton talks about what it takes to create a "nerdcore" song (I really do not know where that term came from) and what it was like working with Ellen McLain, the voice of GLaDOS.
Why don't more games end in a song?You can read more of the brief interview courtesy of CVG.
Jonathan Coulton: We know how to tell stories in movies, but we're still learning how to do it in games. And that's why this marriage of song and game succeeds: by the end of the game you've developed a relationship with a very complicated character. So when she starts singing, it feels almost natural.