I dont know if you've visited Google Image Search's homepage recently, but there are a few pictures underneath with a little sentence encouraging you to drag the pictures onto the search box, as a "new way to search".
one of the pictures is a painting by van gogh, and when you drag the picture onto the search box -- lo and behold! -- it "knows" what it is and brings up a load more.
I dont think any of us would be surprised by that, but i was bored so i did a little test where i dragged a thumbnail image of charles dickens into the box, straight from the hardrive on my computer (not a website), with a gibberish filename and all the metadata stripped out, and it still knew what it was! at the top of the page it said "Best guess for this image: Charles Dickens"
It was quite a famous picture, and some of the images it brought up where the same image at full size, but even so, i thought that was quite impressive seeing as my thumbnail was tiny (80px by 60px) -- and it was cropped.
Can google honestly tell what a picture is with no textual help?