This question has been brought up on this forum before, the bamboo diapers all say rayon/polyester now (I think at first they said bamboo then it was changed). I seem to remember that Mother Ease's response, when asked, was that the diapers are made with bamboo fiber, but because of the chemical process used to process the fiber, it has to be called rayon (and it is mixed with a bit of polyester to make it stronger). Or something like that. I know the question was brought up again more recently by someone saying that rayon by definition could not be bamboo, but once again I can't remember exactly why. I think it would be worth asking Mother Ease if you have any doubts...
I know my "answer" doesn't answer much... Hopefully someone has better memory than me or can find the old posts for more information!