Sadly, O'Connor has not resigned. If she had resigned there would be a vacancy. As it is there is no vacancy.
She has handed in a resignation that she says is effective on the confirmation of her replacement by the Senate.
Effectively, she has thumbed her nose (again) at the Constitution which gives the president recess appointment power to deal with any Senate's inability to effect a timely consent to a nominee. With her non-resignation and non-vacancy, she has very "cutely" taken that option away from the president.
Perhaps the president would never have used it, but her disdain for the constitution includes making herself bigger than it's provisions and intent.
Were I president Bush, I wouldn't have accepted her letter of non-resignation.
Wonder if she could pull her letter back if she didn't approve of the nominee?