President Obama made the announcement today in the Rose Garden, appearing with Gen. Petraeus and VP Biden. Video in the previous post.
Update 1: On Petraeus, Pres. Obama's message is that this is a change in personnel, but not a change in policy.
Update 2: It's over. The staff has changed, but the policy remains the same. That means the debate over whether or not we should be in Afghanistan will continue, but by replacing McChrystal with Petraeus, Pres. Obama has cut off the ability of Republicans to try to score political points with McChrystal's dismissal.