I am an early education teacher who has been providing family chilcare in my home for the last 12 years. Between all the little ones I have loved here and my own two, allow me to put in my two cents worth on this. 12 mo olds, actually young children in general, are the original scientists, always interested in cause and effect. I don't think this problem is rooted in anything deeper than this. What happens if.... I would encourage you, an anyone else experienceing this behavior to consitently and calmly to continue with firmly holding his hand and saying "no" sternly when he hits a face. Back this up with setting him down if he does it a second time. As well as teaching him limits, you are teaching him that it is OK for people to set boundaries on how they allow other people to treat them. Obviously he won't catch onto this "sub-plot" at 12 months, but this is the beginning of you modeling this important for him. The most important thing is to be calm and consistent as he learns what is appropriate and inappropriate.
Additionally, for the children in my care, I feel that slapping anyone anywhere on the body is inappropriate. No one needs practice in hitting. When toddlers slap others, I hold their hand and say "gentle touch" as I demonstrate with their hand just how to gently touch another person. This wild and wacky hitting is also related to the fact that children this age are still learning how to control their bodies and the appropriate amount of energy with which to touch another person. I am more strict on touching the face though, since our faces, eyes, are particularly sensitive to pain. For me its a matter of self-defense since I wear contacts and don't want to have one lost.
Hope this helps. I had my children late too. My son is now 20 and doing very well at college. Parenthood has been the best experience of my life. It goes very quickly, so enjoy, enjoy!
K.