Similar Images is graduating from Google Labs and becoming a permanent feature in Google Images. You can try it out by clicking on "Find similar images" below the most popular images in our search results.
It worked both as a disambiguation tool and as a pattern analysis tool... and the pattern analysis could be skewed by color. Isolating the search (via photographer's name) to black and white images with no defined subject gave a better idea of how the pattern analysis worked....
The search considers a combination of meta data and image analysis. Results are amazingly good, but not perfect.
To check out the image analysis by itself, I tried searching by the name of a former Life magazine photographer whose work is all in black and white, and areas of light and dark were initially big factors in what similar images were returned. I could keep refining the search, though, just with the "similar" link, until Similar Images zeroed in on multiple copies of the same photo.