After testing it out in Google labs for 6 months Google is now launching similar search as a permanent feature in Google images. This is how it works:
“Let’s say you want to find images of Ancient Egypt. Google Images will provide you with a rich variety of results, including pyramids, maps, relics, drawings and other types of images. Instead of poring through hundreds of images, now you can simply click “Find similar images” to narrow down the results to the results to the type you want. (We’re rolling this out gradually, so the links in the below examples may not work for you yet.) “
It’s unclear from the Google announcement what type of technology is used, i.e. wether it’s visual search, keyword based or a combination of the two. If anyone has more insight feel free to comment and contribute.