On CSPAN Live. Watch online here (paste into Windows Media Player if it doesn't open directly).
I will update this later with more details.
Maria Shriver, first lady of California (yes, Arnold Schwarzenegger's wife) was introduced by Michelle Obama by surprise at the rally in Los Angeles, California at UCLA, along with Oprah Winfrey and Caroline Kennedy.
UPDATE: Ok, I realize there are way too many diaries and posts on this right now. To quote myself in the comments:
Could there be enough breaking diaries on top of a front-page post? This reminds me of what happened with Al Gore's surprise endorsement of Howard Dean. (Yes, I've been around that long. Check my UID.)