I hadn't heard any of it until about a week ago when I saw it in a comment on some random blog and started doing some digging. Apparently, it's been a longstanding thing that they don't like each other and many of the things I read used the word "hate." I love Kristin and don't want to imagine her being like that, but I can also imagine her coming into that show (Wicked) being promised a big part that would get her lots of attention and, potentially, a Tony, only to see more of the attention (and the Tony) go to her lesser-known co-star.
It might have been Michael Ausiello's blog that had the blind item about them hating each other and refusing to act together on Glee. Well, since it was a blind item, it didn't name names or shows, but it was pretty obvious who he was talking about.
Kristin has done a ton of stuff since Wicked but, you know, Idina married Taye Diggs, which makes them at least even in my mind!