Margaret Cho

Someone needs to keep the Palin girls off Facebook -- this time it's Bristol posting an open letter to her "Dancing With the Stars" co-star Margaret Cho ... and making a gay "joke" at Margaret's expense. Bristol responded to a blog Cho wrote this week that suggested Sarah Palin made Bristol do…

TMZ is blowing the lid off of the "Dancing with the Stars" premiere -- 'cause we know EVERY SINGLE THING that's set to go down on the show ... from songs to props and even EXPLOSIONS! BTW -- SPOILER ALERT follows: The most shocking performance -- Bristol Palin and Mark Ballas -- they Cha-Cha to…