i was at the start of a Critical Mass in, of all places, Honolulu, Hawaii when it happened to one of the activists. He had 2.5s on his Surly Instigator. Big huge knobbies. Thebike was just leaning against a planter when "BLAM!" it exploded.
Unlike the original poster, nothing was damaged too bad. The innertube just came hemoraging out of the tire — kind of like a hernia.