See http://oxforddictionaries.com/definition/english/Godwin%27s%2Blaw .
I found out about it this week when @OEDonline tweeted this:
MT @OEDonline: OED Word of the Day: Godwin's Law, n.: http://t.co/rwi5f3kIm1
Now if only it were in the print edition.
--Mike