> Now that we have confirmation that Bing uses clickstream data on sites that they do not control, I'm surprised that no one mentioned this makes them sensitive to click-fraud.

I would expect that they know this. The question is how susceptible does it make them? The clickstream is just one of something like a thousand factors that go into the ranking. Perhaps on any real page, it would take more fake clicks than a fraudster can generate to make a worthwhile difference.

Keep in mind that Google was experimenting with pages for which Bing had no information other than the clickstream, so the clickstream made a big difference there.