File this under; "I think this is new".
Google's now telling me when safesearch is on or not for normal text SERPs. I imagine this becomes more of an issue as Universal Search ramps up and the basic web search starts to include more images and video.
alternatively, I was doing some work for a cosmetic surgery client on Friday and flicked the safesearch settings on Google Image search on and off a few times to see the difference (wow; what a difference!) and so the appearance of this safesearch banner may well be due to that!