Picassa got this back in September. iPhoto has it now too.
Note that these are radically different, as Picassa has it in both there web based and client side software, while I believe iPhoto is client-side only (though reading the tags off your Facebook photos and importing them back sounds pretty slick).
Windows Live Photo Gallery has been rumored to be getting this since July, though I don't know anyone who uses Live services besides Hotmail.
Flickr has a slightly unusual position though. There are plenty of professionals who upload their art, while there are also lots of consumers who just dump their images there (count me in the latter). As far as I know, Flickr is a losing proposition for Yahoo ($25 for unlimited image hosting doesn't allow you to make a profit). In other words, there's not much reason for Yahoo to invest into R&D for facial recognition, except to stay competitive in the photo sharing market . . .