I've just come off BA294 from O'Hare which had come via Houston. Flew with 5 American guys who were on the Houston flight which had the engine fire. From their description I would say this was a pretty major incident - they could see flames from the engine, there was dense smoke in the passenger cabin, and there was an immediate evacuation down the emergency chutes upon landing. Apparently the BA crew were exemplary - handled the situation extremely well.