Found this where she talks about 'Stairway':
Stairway to Heaven" (Jimmy Page/Robert Plant)
It was a song I loved and a song that [husband] Carl loved. We used to love it together. To me, it's like "After the Goldrush." It's an abstract song. You really don't know what it means. It sounds Old World. On the last couple of albums, I did "Shine," "I Get a Kick Out of You" and "Traveling Prayer," so everybody is expecting me to do something "different" on this album, too. Well, I'm the only person to have the nerve to deal with classics. That's why I took it, ad-libbed it and made it more spiritual.
I didn't do it just for the gimmick - my love for the song comes from a very real place. It's not just about making it work - it's about it really being a part of you. So I thought, "What the hey - I'm just going to go with it, and if it didn't turn out good, nobody would ever know I even tried it." I knew I was walking on sacred ground because it is a classic.
But any time you make a change in lyrics or in my case the ad-libs, you have to get an OK from the writers and publisher. I was scared to death to send it to Robert Plant and Jimmy Page. They sent word back that it was fine and they loved it. In fact, Robert Plant said he'd always thought of it as a spiritual song, and he was thrilled we'd used a choir on it, because he thought about that, too. If they like it, that's most important to me. But I do hope the public will accept it too. I even hope they love it.