Hugh - you make a good point. I would still pose the question less directly and try to find out what the circumstances surrounding the creation for the song. That would probably open up the conversation more than a direct(and sometime rude) "what was that song about"?

I've had converstation I've told the songwriter what the song meant for me - images or feelings that it invoked.

On the other hand, if the songwriter was LOOKING for honest feedback, I would tell the truth - I had no clue as to what that song was about.