David Pennock and Daniel Reeves, cited by James Surowiecki (author of The Wisdom Of Crowds) in a comment on Midas Oracle:

[...] Clearly, for most people, averaging their own assessment with others is the right strategy. [...] As for how to aggregate probabilities, the above analysis assumes a simple arithmetic mean and we have confirmed empirically that this choice beats geometric mean, root-mean-square, and every power mean in between, regardless of the number of probabilities being aggregated. [...]

I would say “30%”, Jason Ruspini would say “50″, Steve Roman would say “40%” –> the averaged probabilistic prediction is 40%.