Is rebirth a conventional truth within the Buddha's message?
Rebirth is very central to the whole doctrine. As long as this Kammic force exists there is re-birth, for beings are merely the visible manifestation of this invisible Kammic force. Any attempt is like excluding Samsara is like excluding this from the whole equation.  Birth precedes death, and death, on the other hand, precedes birth. The constant succession of birth and death in connection with each individual life flux constitutes what is technically known as Samsara -- recurrent wandering. Buddha said that:


  "Without cognizable end is this Samsara. A first beginning of beings, who, obstructed by ignorance and fettered by craving, wander and fare on, is not to be perceived." 


The cause of this Kamma, continues the Buddha, is avijja or ignorance of the Four Noble Truths. Ignorance is, therefore, the cause of birth and death; and its transmutation into knowingness or vijja is consequently their cessation. The result of this analytical method is summed up in the Paticca Samuppada. Paticca Samuppada is the discourse on the process of birth and death. 

Nibbana is often conceived of as stopping this cycle. By removing the causes for craving, craving ceases. So with the ceasing of birth, death ceases. With the ceasing of becoming, birth ceases... and so on until with the ceasing of ignorance no karma is produced, and the whole process of death and rebirth ceases.