I can see Jack buying Michael's anxiety over Walt. He wants to have a plan and stick to it - he's scared about having too many unknowns. Everyone knows how concerned Michael is over Walt, how it's made him a bit unstable, and I think it's perfectly reasonable to assume the other castaway's are going to give him some slack. It's only when Michael actually asks Sayid to back out, when he is the most logical choice to join, that a red flag really goes up, and Sayid was the only one to witness it.
Also interesting that Kate is on the list, after the Others had given her back once, and Jack's previous comment that he took her with him to make the trade deal because "they don't want you."
At the Other's camp, did Zeke look meaningfully at some tent and say something about asking "her?" Don't remember the exact quote, but it seemed to imply he was referring to a woman who's his superior. My first thought that it's going to be Rousseau as a surprise twist, but I can't see that making any sense, given her behaviour previously in the series.