Okay...here comes a big spoiler. I can't say who I got it from as I would be in big trouble, but it is from someone in the entertainment field.
Please do not quote the spoiler if you are going to reply to it as if some people are using a different color forum theme than me it will show up even if it in spoiler brackets.
Click to see Spoiler:Maggie Grace (Shannon) - has been released permanently from the show.
ETA: Due to my friends work they have met every cast member from LOST so I am 99 percent sure this is true. Leaving room for the one percent so that I can hurt him if he is wrong :)