Hillary CLinton is not giving up. She is not admitting defeat, nor is she conceding the most coveted prize in American politics to her rival Barack Obama. Clinton's long-awaited withdrawal is merely a political maneuver aimed at saving face while she continues to pursue the Vice Presidential slot on the Democratic ticket.
That's right, I said "continues" - for I refuse to believe that Clinton did not already recognize months ago that she would not be nominated for President. Hillary Clinton, in an act of brilliant political strategy fought hard to prove her necessity; to show the Democratic party that they need her in order to win this November. Hillary has already begun her campaign for VP, a fact that is obvious to anyone her saw or heard her speech last night in which it would have seemed that she was declaring victory rather than defeat.
Yet, Hillary's attempt to achieve VP will soon be brushed aside for it would be political suicide for Obama to grant her that level of closeness to himself. Firstly, Hillary's supporters and Obama's do not and never have sen eye to eye on anything. Obama runs on a platform of change, whereas Mrs. Clinton dredges up bad memories of scandal and political doublespeak. By granting Hillary the VP slot, Obama would effectively cancel out the votes of his own supporters.
Secondly, if Obama truely wants to prove his ability to lead, he will not allow the Clinton dynasty to overshadow his presidency. As she did when her husband was president, Clinton will attempt to interfere in all aspects of the presidency. It would be the first time in history that the president answered to his VP - a fact that Obama recognizes and fears.
It is for these reasons that Obama will not choose Hillary Clinton as his running mate, and it is for these reasons that the Democratic party cannot yet declare itself united, for the battle for power has merely been clouded over but not yet erased from the public eye.
EDIT: I just read this Dick Morris article which basically agrees with me but throws Bill CLinton into the mix.