Good article from Foreign Policy that runs down the possible implications of today's vote. In short:
"It's not simply that the outcome is unpredictable," Maloney continued. "It's that the impact is not wholly straightforward. You could have a reformist win that revives a power struggle that returns the Iranian position on engagement to one dominated by paralysis."
In other words, it would be good for the U.S. if Ahmadinejiad loses, but it won't necessarily be all wine and roses if Mousavi wins.