I have no issue with my parents or my close friends (who know my kids well) disciplining my children. Whenever my parents stay with us I tell them to feel free to growl, put them in time out etc if the boys do something to deserve it. Luckily for my kids, my parents are softies!

I have a couple of close girlfriends who I've actually asked to "speak" to my kids if we're at their house. Half the time my boys won't listen to me when they're asked to behave, but the minute one of my friends uses a stern tone and asks them to behave, they snap into line so quickly it's a wonder their necks don't break.

I'd draw the line at people I don't know well telling my kids off (and I'd be extremely unhappy if they smacked) ... but then again, my boys are pretty good when out and generally don't give people (outside DH and I) reason to get cranky.