Actually, Clinton getting out of the race would be very bad for Obama right now. He'll need her supporters in the fall; that being the case, he needs the legitimacy of his nomination to be absolutely ironclad, airtight, and unassailable, so that no one feels Clinton was forced out of the race unfairly. Having her stay in the race through the end of the primaries is vital to that.

Clinton herself knows she's lost. That much is obvious--you'll notice she's stopped taking shots at Obama and has started concentrating fire on McCain and Bush instead, not to mention talking about the importance of Democrats coming together in the fall. She's staying in the race because it's good for her as well as Obama. By sticking it out, she comes out looking like a fighter rather than a quitter. Plus she gets some extra leverage with Obama when it comes time for them to sit down and hash out a deal.

She actually is supporting Obama at this point. She just hasn't come out and said so yet. Once the last primary is over and Dean badgers the supers into making up their minds, she'll concede, but not until then.