If O Brother, Where Art Thou? cracked open the door to bluegrass' past for a new generation, Nickel Creek are bent on giving it a future with Why Should the Fire Die?. Boasting instrumental chops and three-part harmonies worthy of a mountain-soul string band, this coed trio is steeped in tradition. But their third album also flirts with pop immediacy. ''Helena'' fine-tunes their strengths, as acoustic delicacy gives way to a sublime fiddle solo, then builds to a furious punctuation point.