I see that this is an old question, but the answer at that time is the same as now. Unification is just a myth, better question is why Robert Plant doesn't want to reunite with the band! But that has been answered, he doesn't see Led Zeppelin without Bonham. Although his son did a great job filling in, it's just not the same!
So the answer to this question is that the band will never reunite, Plant doesn't want to, Bonham is dead and the only unification that might happen is with Page, Jones, Bonham's son and a new singer. But they tried that and they saw they can't find a replacement. So again, it will never happen.