I just saw one for the first time: http://bit.ly/No8A5Q I forgot to screenshot the 'thank you for your feedback' screen but it was from Google. Also I was using Firefox, signed out of a Google account that doesn't track any history.
Ha. That's funny. I never encountered any feedback by Google before. Maybe it could be caused by some plugin that you installed. Or maybe it could be something genuine.
I've never seen it. It could be something they're testing or it could be caused by one of your plugins. I think Google would be more likely to use implicit feedback - whether you clicked on a result - as a measure of your satisfaction with the SERP rather than asking you, but who knows, they are always testing many things.