Can I tag Faces on iOS using the iPhoto app?
I'm afraid not. You can do places and whatnot from your i-device, but sadly no faces ability, although I have the feeling Apple will do it in the future. You can quite easily do it on iPhoto however. If you need any help in getting started at all in iPhoto, here's Apple's own support page: http://docs.info.apple.com/article.html?path=iPhoto/9.0/en/pht2297cf5f.html

Hope this helps.