Finally, they did it.
I was waiting for it since months, when it had been annouced with previous version of Picasa : google implemented name tag for faces in Picasa.
Why is it so important for me ? Because now as google says,
" I can organize your photos based on what matters most: the people in them "
Basically, picasa (desktop) scan your photo and find faces.
then you can tag them (like on facebook for example).
The algorithm seems to be efficient (of course, between kids... they're some mistakes) and the functions to tag well implemented (photo / photo, bulk update, suggestions, ...).
I'm using it since yesterday to tag my thousands of photos. I'm very happy with it.
You should try it !