Aside from her passionate insistence that her supporters get behind Barack Obama, I think this might be the most important line in her speech:
"Our lives, our freedom, our happiness, are best enjoyed, best protected and best advanced when we do work together. That is what we will do now, as we join forces with Sen. Obama and his campaign. We will make history together as we write the next chapter in America's story."
In saying that it's time to "write the next chapter in America's story," Hillary was using Obama's own language, words he's repeatedly used to describe his own historic Presidential bid.
In a sense, this was perhaps the ultimate concession: Presuming Obama wins the White House, she acknowledged, the next chapter in America's story will not be the one she intended to write, but the one Obama is writing.
And she's now going to help him write that next chapter. It's really the most powerful message she could have sent to her supporters: It's not our time; it's theirs; and as difficult as it may be to accept, we're going to help them make it happen.
Late Update: Come to think of it, that might be the real significance of this line from her speech:
"I am standing with Barack Obama to say, `Yes, we can!'"
"We," according to Hillary, now comprises Obama's supporters and hers.
It needs to be said that Hillary struck an extraordinarily difficult balancing act with real grace and eloquence. On the one hand, she needed to signal that she has built a movement of her own and to reinforce the idea that she is the undisputed leader of American women -- both as a genuine point of pride and as proof of her undiminishing influence. Hence the repeated references to the 18 million votes she earned.
Yet she needed to do this while signaling unequivocally to her supporters that all the energy and passion she's unleashed now has to be channeled towards delivering the prize she and her supporters coveted with such intensity to someone who has been her bitter rival for nearly 18 months.
And she pulled it off. Really an extraordinary performance.
Late Late Update: The front page of Hillary's Web site now features the Obama line I flagged above...

Late Update: Obama responds.
Late Update: I wrote above that Hillary was trying to "reinforce the idea that she is the undisputed leader of American women." I didn't mean to imply that she does in fact occupy this role; merely that she is trying to advance the idea that she does.
Nonetheless, I should have written this differently. What I meant was that she is trying to reinforce the idea that she is the undisputed leading woman in American politics.