Crikey - I didn't realise what I was starting with this thread.
My take is that it doesn't amount to theft, but is against the rules. However in all of my many visits to the DC lounges I've never noticed the signs stating that food/drink needs to be consumed within the lounge.
Those who believe it is theft should perhaps just pray for the souls of those of us who take the occassional bottle of water or packet of shortbread.