I never understood the usefulness of the preview feature. The only time I find it marginally interesting is when a website is currently down or had recently been taken offline. Though, I find the "cache:" query more useful in those cases anyway.
The preview window does pop-up unexpectedly for me as well. I guess my mouse pointer wanders over to the right side of the search result every once in a while, triggering the preview.
I think the preview thumbnail is a very poor substitute for the 'cached' links that have disappeared. Sometimes the cache link was the best thing for an underperforming website or one that was down. It was simple, everyone understood it, now it's gone. Boo.
The preview window does pop-up unexpectedly for me as well. I guess my mouse pointer wanders over to the right side of the search result every once in a while, triggering the preview.