I'm not trying to start a war amongst the fans but lets face it Izzie has been taking her digs at Callie every which way she can for quite awhile. In previous episodes we've seen her constantly make fun of Callie. It's akin to watching one of those Teen movies (like Mean Girls for example).
What has Callie ever done to Izzie? Nothing really. All Izzie has ever done to her is be mean and incorrigible. And to top it off she's fornicated with Callie's husband. TROLLOPING TRAMP!
Oh yes, that will go over quite well with the other fen.