Thanks guys. I think making that distinction between honesty and privacy is one that I was failing to observe. It seems that most of your answers are in the same vein.
In fairness, I had no pre-conceived notions about what might or might not have been considered a secret in any future relationships. It just struck me that this was a logistical question I had failed to address when trying to work out exactly how I would want a poly relationship to go.
In any case, this is still all hypothetical at the moment. We're still in the having-difficult-conversations stage of transitioning from mono to poly, but considering questions of this nature now will make those conversations that much easier later on.
Thanks again all!