I checked in the code responsible for doing that :
into the project's SVN at code.google.com. I'm really excited. This is so cool. You can try it at home by checking out the code from svn and compiling it. Remember that the hack lives mostly inside libfspot (and the UI part in f-spot itself), so you need to build it! A word of caution - do not expect this to be anywhere near opencv, the detection algorithm bases on classifiers that have been trained on a very limited set of input data (I'm kind of in a rush), and the training algorithm itself needs some more love. But the results are promising, I think. I'd use some community input on how to handle the UI integration, I'm currently swinging something very rough, just to get basic functionality.