Polyamorous cows, Paris Hilton, and the OED

Now that I have your attention... check out the latest additions to the OED:
http://dictionary.oed.com/help/updates/pleb-Pomak.html

Some of these terms have been around awhile (like polyamorous and cow-tipping) but are just now being added, while others are fairly recent (e.g. celebutante -- thanks, Paris Hilton). I'd love to know how they decide when a word is sufficiently ingrained in the language to warrant an entry.