it's in google's best interest to get studies to show them with lower market share. I'd bet if you tallied up the stats for 100 biggest sites, Google would have at least 90, if not 95% of all search engine traffic
I doubt that just for the fact that IE comes with bing as default. I don't think enough people actually care to change it. I would not be surprised to see it higher, but I highly doubt greater than 80%.