In Ajahn Sumedho's book The Sound of Silence there is a chapter named, "Questions About Awareness and Rebirth" (http://books.google.com/books?id=TqKbrm ... #PPA237,M1) where Ajahn Sumedho states:
Do you guys agree with Ajahn Sumedho's view that rebirth is speculative? If you do or do not, then why?"Rebirth," like "reincarnation," is a term that's used generally referring to having gone through a series of different lives, and then there are various views about whether once you get reincarnated into human form where you can go, become a frog again or something like that. . . . But the truth of the matter is, nobody really knows.
The historical Buddha refers to previous lives in the scriptures and things like this, but for me these things are speculative.