The bratty children are usually ones that aren't being disciplined well.
I know you used the word "usually", but being a parent, and knowing lots of other parents, out in public is when MANY kids can get very bratty. Even otherwise well behaved children can get overtired, overstimulated, and hungry can be very difficult to deal with and not being well desciplined at home has nothing to do with it. They are not little adults, and are still learning how to appropriately handle situations that are uncomfortable to them.
Believe me, we feel the condemning stares of those, especially childless adults, that have " do SOMETHING about your bratty kid" written all over them. Though we try to anticipate those occasions, sometimes unexpected things do happen and we are not prepared.
As far as spanking goes....
An occasional swat to an othewise well loved, and well care for, child isn't going to cause any permament psychological damage. I do try to avoid it, however. The taking away of favorite toys and tv shows seems to work better in my house.
Indeed, like Princess said, that's why Game Boys and, in my daughters case, portable DVD players were invented.