First of all, their incentives are not aligned. Hillary Clinton doesn't want to see him win; she'd like to see him lose with herself in the VP slot -- making her the presumptive frontrunner (once again) for 2012. If they would win, Barack would have to put up with eight long years of power plays, backroom betrayals, blatant opportunism and backseat driving (games at which the Clintons are masterful). Ask Al Gore how that all worked out during Clinton I . . .
At this point, having lost the big prize, Hillary Clinton must figure she's in no-(more) lose situation. Publicly pressuring Barack, through surrogates, for the #2 slot means either that she gets it, or that his standing is further damaged among her followers when he denies it to her. So she's either positioning herself as the heir apparent if/when Barack loses, or actually enhancing the likelihood that he will lose through the defection of her constituencies.
If I were Barack, I'd just pick one of her prominent supporters and leave it at that. How ironic for Bill Richardson (who may have endorsed Obama partly in the hope of getting the veep slot) if he would more effectively have enhanced his chance of getting on the ticket by having endorsed Clinton . . .