Is rebirth a conventional truth within the Buddha's message? There is a lot on this site (and elsewhere) on the validity of rebirth, whether it is necessary to understand the Buddha's message, or to reach enlightenment etc. My question is slightly different where within the two truths doctrine is rebirth placed by the Buddha?

I understand that 'the two truths' developed a little later on, but in that later developments within Buddhism were and still are heavily influenced by the Buddha's words (Dhamma and Vinaya), is it possible to come to an agreement on whether rebirth deserves to be considered an ultimate or conventional truth?

Here's my thought: rebirth is conditioned by death and the workings of Kamma, therefore it seems to fit into conventional truth pretty neatly. It also makes no sense from the ultimate point of view - no conditioned experience can either be affirmed or denied within the ultimate standpoint, so whilst rebirth does not make 'positive sense' from the ultimate stand point, it also can't be denied altogether (that is to say, ultimate truth is empty of rebirth).

Is rebirth a concept that cannot be affirmed when considering ultimate truth, but cannot be denied when considering conventional truth? 

(edit: I assume rebirth occurs and is evident. The fact that 'every thing that has a beginning has an ending' demands rebirth if avoiding annihalationism)
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.