I was wondering the same thing. I would bet she wasn't, just because of how tight her skirt is. There would be no way to avoid a panty line...unless the fabric is pretty thick.
Ever heard of a "thong?" And since when are panty lines grounds for throwing someone off a plane? No, another poster had it right: there's much more to this than what we're being told.
Was she wearing undergarments?
Not sure about on the plane, but as I posted early, she was in the Today show studio, because when she sat down, she gave the audience a nice crotch shot....silky white panties if I recall :)