Or it could be a sure sign that Rove won't be indicted, so Bush can pick someone else who he won't have to worry about being tainted by Rove's indictment :)
I don't think this withdrawal has anything to do with Fitz, but, there's no better way to rally the troops in the face of adversity than to nominate someone like JRB. When you lose a battle, you want to shore up the core of your support.