No, his position was temporary and he needs the Senate vote to make it more permanent. His resignation letter shows that he has no interest in dealing with the obstructionists again. Out of respect, Bush honored his request.
Stephanie, I don't and wondered the same thing. Your point that it could be a sign of more cooperation is certainly possible but I don't think that changes his level of support for Bolton. It may just imply he is making a wise decision.