As far as I'm aware of the data, Bing's mostly been eating non-Google market share, e.g. Yahoo, rather than denting Google significantly. Back in 2008 Google was also at 64%, but Yahoo was at 20%. Now it's Bing.
Yup. I don't know if anyone researched it, but I bet it highly correlates with browser usage. On a couple of websites I run, I notice that IE market share is much higher in the US than Europe.
This is surprising, I guess having it has an IE default really has more of an impact then I expected. I wonder how much the amazon gift card thing helped as well.
Not really. Before Bing had 20%, Yahoo had 20%. Now Yahoo search is powered by Bing. MS pretty much bought the marketshare with the Yahoo deal and Yahoo was happy to have someone else do the heavy lifting on search. I believe Yahoo search is also the default on Firefox now.