Hillary can't allow Obama to win this election because she would be 69 years old after 8 years of Obama.
Her last best shot would be to go after Palin if McCain does not seek a second term or an even older McCain if he does seek a second term.
Of course, this supposes that she still wants to be President of the United States.
By the time 2012 rolls around I doubt Hilary will be a factor.