UPDATE: I should point out that this obviously raises the stakes to "All In" for Hillary in Iowa. I've mentioned before that the third-place finisher in Iowa will look an awful lot like the last-place finisher in Iowa. Some argue that with Hillary's money, she can afford a weak showing. This endorsement confirms that her campaign doesn't believe that, either.
MSNBC.com says: BREAKING -- AP: Officials say ex-Iowa Gov. Vilsack will back Sen. Clinton in '08 race.
The timing on this is ... interesting (at least they waited almost a whole 24 hours since the Edwards news conference). The timing certainly makes it look like the Hillary Clinton campaign is very, very worried about the John Edwards campaign.
My feeling all along was that Hillary was desperately hoping Vilsack could take Iowa off the table. When it became clear that he couldn't do that, I knew he would be endorsing her.
Don't expect much boost for her out of this. Vilsack's numbers in Iowa, such as they were, were out of a sense of loyalty to him. It's not transferrable.