I'm aware of the wisdom of crowds phenomenon -- I believe sixesandsevens used a good strategy. (Yes.) I haven't read about the corrections, but I wouldn't be surprised if there are some ways to optimize the process. (Why do you ask this second question?)

I wonder whether the correct mean for this sort of wisdom of crowds averaging is arithmetic or geometric.

I think this is an interesting question too. Perhaps someone knows the math for this; alternatively, it would take a few minutes to test it with some made-up guess distributions. (Any takers?)