Here's a link to the cast pictures:
The following are quotes from the message board where I got think link:
What I've read on that is that all former champions were automatically invited back (w/ exception of this season, having Rupert there is not a spoiler), so that's why Jenna is there. I predict that Jenna will not do as well this season. I can't imagine a Tina or a Jerri letting her get away w/ being under the radar again. Brian from Thailand and Days of our Lives and Porn apparently has a bad relationship with Survivor and that's why he's not there and Shii Ann is. Alicia from Australia and Rob from Marquesas are apparently the two who were on the bubble -- if more exciting people had accepted their invitations they wouldn't be there.
The Shii Ann puzzle answered above. Vecepia turned it down. Not sure on Heidi or Neleh. Elisabeth doing the View plus her other show, can't film (it's filming soon or even now). Roger and Pascal were asked but both felt they were a little too old for another go-round.