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Never seen this before - anyone else?
At least that's my experience. When folks reach my page through regular SERPS they tend to stick around. Those users searching for a general term for which an image from my site appears, but which would not have brought up my listing in the regular SERPs, tend to back out right away.
I don't think my ad impressions are paid when they show up in the G cache, so these hits on my pages are a waist of bandwidth for me.