I am building a game and would like to reuse a song from the Portal 2 soundtrack.
But while it's free as in free beer, I can find no information on licensing.
What are the licensing terms for reuse of the music?
Questions on Arqade are expected to relate to gaming within the scope defined in the FAQ. Consider editing the question or leaving comments for improvement if you believe the question can be reworded to fit within the scope. Read more about closed questions here.
Its under copyright and they do not have any posted licensing terms, so unless you have a reason to claim 'fair use' there are no licensing terms.
If you want to license the music and include it in your game, you are going to have to contact valve. http://www.valvesoftware.com/contact/