Brian here again: I was playing this video (which is a U2 song reconstructed from a State of the Union address, for those few out there who wouldn't know that--you know who you are*), and Iain thought it was very funny! He laughed at it for quite a while, then all of a sudden, with a minute or so left in the song, he got very serious. Was it something he said? Perhaps it was the profundity of Bono's lyrics--perhaps Iain thought, 'You know, that is so true.' We'll probably never know.
(* - E.g., Susan)