I found this site from Uncertain Principles (also rated PG, now). Two of the blogs I read regularly are NC-17. I guess it's a good thing I'm an adult. Now I'm all curious about how the rating actually works. It only takes a few seconds - a quick scan of the front page for naughty words is my guess.
Update: Apparently I'm a complete idiot. The reason for the rating is listed below the rating "stamp". I have used the word shoot ten times (hmm, because I participate in Scalzi's Monday Photo Shoot, perhaps? oops, now it is 12) and the word zoombie once (zoombie is PG? now it is up to 3, brahahahahaha).
Update 2: It is hard to look up a word (particularly a foreign one) when you know how to pronounce it and the meaning but there are multiple possible spellings. Damn the French and thier silent 'T's.