Team Obama touts a big endorsement at 7 PM. The endorsement is:
a. John Edwards
b. Al Gore
c. Roger Clinton
d. A cheap way to buy out a media cycle
More seriously, I figure the pecking order looks something like this:
1. Hillary Clinton
2. Al Gore
3. Colin Powell
4. John Edwards
5. Prominent Senator or Governor who had endorsed Clinton
7. Chuck Hagel
8. (tie). Nancy Pelosi and Jimmy Carter*
9. Jim Webb
10. (tie) Brian Schweitzer and Steve Beshear
* Points deducted because of their tacit endorsements already.
Keep in mind that the last two times the Obama campaign teased a "big" endorsement, they turned out to be Lincoln Chafee and Joe Andrew, who would rank about 493rd and 88th on this list respectively.
EDIT: I have no idea whether it's John Edwards. But remember the two contradictory viewpoints that I expressed last night. On the one hand, the Obama campaign knows that the 48 hours following West Virginia were going to be the most vulnerable time in the remainder of the primary cycle for them. So it would be a good time to hijack a media cycle. But, on the other hand, it would be a little awkward to roll out an "in your face" sort of endorsement the day after Clinton won a primary by 40 points. If, I don't know, Dianne Feinstein flipped to Obama, that might trigger exactly the opposite of its intended effect amongst her supporters (see also: the NARAL endorsement). So what you're going for is awe rather than shock.
John Edwards is perhaps the only name that can deliver awe without shock. And that's because, if you look at his appearances on Morning Joe and Larry King Live, he's been softening the ground on a potential Obama endorsement for about a week now. And he deferred to Clinton until after the North Carolina primary and Obama won that primary. It's an endorsement that would gather lots of headlines, but that wouldn't give the appearance of being hasty or presumptuous.