With clients I know I'll often use a smiley because we've probably had a laugh face to face so it's ok. On forums I do, in blog comments I try not to but sometimes I worry my reply could be read too cold or formal.
I think on forums it's sort of expected, but NEVER on formal business correspondence. I will occasionally let the odd one or two into a casual client email if I have a relaxed working relationship with them (ie we chat about what we did over the weekend and stuff).
I think the odd smiley is fine - sometimes it helps if you're being sarcastic and you don't think it's coming across just with words. But I wouldn't use one if I didn't know the person well. And NEVER EVER one of these: x