Surprised this isn't already up, didn't see it with a quick search here..
But it looks like it's pretty new news, already on DCW (where they actually say it puts Barack over the top for superdels), and the Obama site itself..
Mitchell's words:
I’m proud to support Barack Obama for President. Senator Obama and I worked together last year to improve care for our soldiers and veterans in the wake of the scandal at Walter Reed, and I know that, as President, he will work hard for our men and women in uniform. Like the primary voters of my congressional district, which Senator Obama carried, I am inspired by Barack’s vision for America, his ability to unify our country and bring much-needed to change to Washington.
No lookin' back, anymore. :->