Blame Norah: Labels are going cuckoo for mellow coffeehouse crooners. First up, Jamie Cullum, a pianist-singer whose Verve debut, Twentysomething (due here May 11), has gone platinum in his native U.K. Currently touring on the jazz-club circuit, the 24-year-old tosses together sensitive acoustic pop and sprightly reworkings of Tin Pan Alley chestnuts. Does Cullum mind the Jonesian comparisons? ''God no! Someone that talented, it's a great compliment,'' says Cullum. ''In fact, I wouldn't mind marrying her.''