you know, thats actually a pretty cool idea. and it doesn't seem that hard, atleast from my (very) limited knowledge on this area....you could just cross reference with images on different websites, like searching for a keyterm.
The server and bandwidth load would be insane.
The server would need to do a pattern recognition on the search-image between all images in the database, plus everyones search-images in total would be huge.

Something like it will probably come, but it will probably take a while.