Jillian Michaels is apparently planning to quit the Biggest Loser after the next season (Season 11) because she wants to have a child. According to the story, her unnamed replacement has been chosen and will work with Jillian next season. http://www.realityblurred.com/realitytv/archives/the_biggest_loser_11/2010_Dec_09_jillian_quitting
I thought the last few seasons the show has been increasingly tiresome and Bob and Jillian both seemed a bit burned out. Still, it's hard to see how Jillian's replacement could be an improvement.