As you likely recall, the Knocked Up actress exited the show midway through Season 6 when ABC released her from her contract. Katherine claimed at the time that she wanted to focus on her burgeoning family, but she had been a vocal critic of the show — and her storyline in particular — and it certainly seemed like there was no love lost between Katherine and Shonda.
But Katherine's film career cooled off quickly thereafter — as evidenced by the critical and commercial flop One For the Money. She resorted to indie films — some of which need financial help — and even agreed to a NyQuil commercial.
Back in 2012, Katherine said that she would "totally" say yes to a return to Grey's, saying she "really, really, really" wanted to know what Izzie is up to — but Shonda hasn't taken the bait (yet).
Considering Katherine's behavior seems a whole lot less offensive than Isaiah's reported use of anti-gay slurs, we wouldn't be surprised if Shonda extended an olive branch one of these days.