Who designs these things. I'd imagine it's going to be something like (II) which is meant to be two arms (the brackets) with a torso in between (II). If someone's fat, I suppose you'd do (O) instead but I'd never do that, because how insulting would that be...
Many people would use this:
>:D<
It's not particularly good as a graphic representation of a hug but it's regarded as 'official' simply because it's the keyboard shortcut to display the 'big hug' emoticon with most messaging services: http://messenger.yahoo.com/features/emoticons/