Most of the people in my group can make a sailor blush. One has a kid that's just started talking, so we're all going to have to learn to be on our best behavior.
Most cursing is OOC, though; rarely do the characters have filthy mouths.
I tend to have a foul mouth, but I started DM'ing a new group of players over the summer and I haven't heard any of them swear at all. So I hardly use foul language in the game out of politeness. I've never mentioned it to the players, I just assume they might be uncomfortable with it.
I only talk like that when I know it won't bother anyone around me.
Almost never OoC. Fighters tend to cuss like sailors. Rogues sometimes use 'ghetto speech' or ebonics.
It also really depends on the players. The ex-military types on the whole cuss more than the civilians as do the ex-jocks. The 'boyz from da hood' moreso that than suburban whitebread.
BTW I'm one of the 'boyz from da hood' and I used to cuss like a mother*****r when I was younger.