This has the potential of being pretty useful to Photogs at tracking down unauthorized use of your images. The database seems a little thin right now.. but it's pretty cool how it works.
I did a test on a picture of the Hollywood sign I took... figuring it would be the type of picture people might use unauthorized. I didn't find my picture being used... but it was amazing the list of photo that looked very close to mine... two of which I actually had to study to insure they weren't mine.
Current version seems to work well... and the browser plugin lets you right-click on an image to find others..
Check it out :
http://tineye.com/