Completely idle speculation here, but Obama's decision to name Susan Rice as UN ambassador and re-elevate the post to a Cabinet-level position can be read as a clear message to Hillary Clinton: Better toe the line, because you can be replaced in a heartbeat. Rice has been a long-time Obama adviser, and early on her name was bantered about to lead State. But there were two knocks against her: inexperience and, to be delicate, a certain lack of diplomatic tact. The UN post will resolve both--as opposed to, say, deputy secretary of state or deputy NSA, which would not have given her the same training, nor put her in a direct line to succeed Clinton. Obama doesn't seem like the firing type, but merely having Rice on hand to take over Foggy Bottom gives him a powerful check against an assertive secretary of state.
--Clay Risen