Only now? What about GMail and how they allow themselves to 'automatically' cruch through it? Google Voice and how it transcribes your voicemails? Google Translate which is trained on everything we put on the net...
I wonder how popular this feature will actually get. From an AI standpoint, its pretty cool. But personally I haven't run into very many instances where if I search for some image, I find it, and now I want another one that looks just like it.
I don't think that's the use case. I think it's meant for when you don't find the image you're looking for, but find one that's close, or at least in the same ballpark. Then you can tell Google to find images that are close and it might find exactly the image you're looking for.
I frequently use http://tineye.com to find images in a bigger resolution. Its also very handy if you want to find the author of a particular image/picture. Their bookmarklet is also extemely useful.
Its never good to have Google enter your space.