I was at the show Saturday night, and here is what happened.
Everybody was in, except for Joel Grey. Anyway, during the scene before Defying Gravity, I noticed Kristen run offstage as the wizard said "And I can help the both of you" or something like that, and Kristen was no longer there. Then, Idina ran off as she was supposed to...and she came up the stairs at the end of the stage (where the attic is.) Kristen, however, just came on from the side of the stage. So I knew something was up.
After intermission, they made the announcement that due to illness, Kristen wouldn't be finishing the show. Laura was taking her place. After the show, we were talking to Laura - she said it was food poisoning (or that's what everyone was saying) and that she was eating buffalo chicken wings when she was called in to fill in the role. At the curtain call, it was great, because Laura carried this caricature of Kristen with her.
I'm as jumpy as a virgin at a prison rodeo!