Is Katherines written in first person? I can't remember.
As much as I love theatre, I think his books in general are too much about internal issues. I'm not sure theatre would work very well for expressing them...
I think it could definitely be produced in a serious manner. John's writing has more than enough humor in it to make most parts funny.
It'd be cool, but there are too many sex scenes for it to be a show. There are some great scenes, but that kind of stuff shouldn't be acted out or anything like that. I'd love it to be a play, but there are just too many of those types of scenes.