I think she has been reasonably mainstream for quite some time now, no doubt helped by the iPod ad. I don't think you can call someone who sells out the Albert hall underground, and that was 2007(maybe 2008 )
She is a fabulous performer and has a wonderful voice, she first came to my attention via The Kings Of Convenience's Riot On An Empty Street album (particularly The Buildup which is a truly wonderful song).
I've seen her live a couple of times, Albert hall and the palladium show a month or two ago. Talented lady and far more interesting than the most of the other dross out there!