I'm thinking ExFJ. She seems ESFJ. Though, ENFJ is still possible.
YouTube - carrie underwood interview
Maybe ESTJ ?
She's never seemed like an extrovert at ALL to me. Here in this interview she talks about how shy she is, and how she doesn't talk much, etc. http://www.youtube.com/watch?v=TnQ0v2WU_Ic
I'm gonna guess ISFJ.
Would it be totally rude of me to say she completely bores me? I always guessed ISFJ with her though, too.
(Not to say I haven't met any exciting ISFJs, I have.)
All people see, but no one sees the same.