All existing accounts have had this preference pre-set to "off" (i.e., you won't see any difference), but it will default to "on" for new users and non-logged-in visitors.
We've also taken a page from the people at /r/pics and added a little CSS to label NSFW posts. This should either help stop you from stepping on a landmine, or help to optimize your browsing experience (whichever floats your boat).
EDIT: please read jedberg's comment for the answer to a question that I know you are all dying to ask... ;)
