Here's the scoop from Michael Ausiello at TVGuide.com.

Do you have any good Grey's Anatomy news for Mer/Der fans?

It appears they've dodged one bullet: I hear Lexie and Derek are not happening — at least not initially. Word around Seattle Grace is that Lexie is being paired with Alex. That should certainly make things interesting when Jane Doe/Ava/Rebecca reappears (possibly) around Episode 4 "The Heart of the Matter."